In ImageMagick 7.0.9-7 Q16, there is a use-after-free in the function MngInfoDiscardObject of coders/png.c, related to ReadOneMNGImage.
### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have verified that I am using the latest version of ImageMagick
- [X] I have searched [open](https://github.com/ImageMagick/ImageMagick/issues) and [closed](https://github.com/ImageMagick/ImageMagick/issues?q=is%3Aissue+is%3Aclosed) issues to ensure it has not already been reported

### Description
<!-- A description of the bug or feature -->
There is a heap-use-after-free vulnerability in function MngInfoDiscardObject of coders/png.c whick can be reproduced as below.

### Steps to Reproduce
<!-- List of steps, sample code, failing test or link to a project that reproduces the behavior.
     Make sure you place a stack trace inside a code (```) block to avoid linking unrelated issues -->
[poc](https://drive.google.com/open?id=1z31pFLCb5tluwW_n_l7Dh9C_YWBs_IvX)
magick convert $poc /dev/null
`=================================================================`
`==21864==ERROR: AddressSanitizer: heap-use-after-free on address 0xf0a2c1ad at pc 0x083e0eb3 bp 0xffafc948 sp 0xffafc938`
`READ of size 1 at 0xf0a2c1ad thread T0`
`    #0 0x83e0eb2 in MngInfoDiscardObject coders/png.c:1569`
`    #1 0x83e1237 in MngInfoFreeStruct coders/png.c:1609`
`    #2 0x84021f5 in ReadMNGImage coders/png.c:7790`
`    #3 0x84fccbf in ReadImage MagickCore/constitute.c:553`
`    #4 0x810a116 in ReadStream MagickCore/stream.c:1043`
`    #5 0x84fbcd1 in PingImage MagickCore/constitute.c:273`
`    #6 0x84fc2e3 in PingImages MagickCore/constitute.c:374`
`    #7 0x8991f79 in IdentifyImageCommand MagickWand/identify.c:304`
`    #8 0x8a26ba8 in MagickCommandGenesis MagickWand/mogrify.c:185`
`    #9 0x8053b31 in MagickMain utilities/magick.c:149`
`    #10 0x8053d72 in main utilities/magick.c:180`
`    #11 0xf60c5636 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x18636)`
`    #12 0x80535a0  (/home/ImageMagick/utilities/magick+0x80535a0)`

`0xf0a2c1ad is located 8365 bytes inside of 12368-byte region [0xf0a2a100,0xf0a2d150)`
`freed by thread T0 here:`
`    #0 0xf7241a84 in free (/usr/lib/i386-linux-gnu/libasan.so.2+0x96a84)`
`    #1 0x808a8e4 in RelinquishMagickMemory MagickCore/memory.c:1089`
`    #2 0x83e12de in MngInfoFreeStruct coders/png.c:1614`
`    #3 0x8401702 in ReadOneMNGImage coders/png.c:7681`
`    #4 0x84021e4 in ReadMNGImage coders/png.c:7789`
`    #5 0x84fccbf in ReadImage MagickCore/constitute.c:553`
`    #6 0x810a116 in ReadStream MagickCore/stream.c:1043`
`    #7 0x84fbcd1 in PingImage MagickCore/constitute.c:273`
`    #8 0x84fc2e3 in PingImages MagickCore/constitute.c:374`
`    #9 0x8991f79 in IdentifyImageCommand MagickWand/identify.c:304`
`    #10 0x8a26ba8 in MagickCommandGenesis MagickWand/mogrify.c:185`
`    #11 0x8053b31 in MagickMain utilities/magick.c:149`
`    #12 0x8053d72 in main utilities/magick.c:180`
`    #13 0xf60c5636 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x18636)`

`previously allocated by thread T0 here:`
`    #0 0xf7241dee in malloc (/usr/lib/i386-linux-gnu/libasan.so.2+0x96dee)`
`    #1 0x80899a2 in AcquireMagickMemory MagickCore/memory.c:488`
`    #2 0x8402117 in ReadMNGImage coders/png.c:7780`
`    #3 0x84fccbf in ReadImage MagickCore/constitute.c:553`
`    #4 0x810a116 in ReadStream MagickCore/stream.c:1043`
`    #5 0x84fbcd1 in PingImage MagickCore/constitute.c:273`
`    #6 0x84fc2e3 in PingImages MagickCore/constitute.c:374`
`    #7 0x8991f79 in IdentifyImageCommand MagickWand/identify.c:304`
`    #8 0x8a26ba8 in MagickCommandGenesis MagickWand/mogrify.c:185`
`    #9 0x8053b31 in MagickMain utilities/magick.c:149`
`    #10 0x8053d72 in main utilities/magick.c:180`
`    #11 0xf60c5636 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x18636)`

`SUMMARY: AddressSanitizer: heap-use-after-free coders/png.c:1569 MngInfoDiscardObject`
`Shadow bytes around the buggy address:`
`  0x3e1457e0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e1457f0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145800: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145810: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145820: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`=>0x3e145830: fd fd fd fd fd[fd]fd fd fd fd fd fd fd fd fd fd`
`  0x3e145840: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145850: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145860: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145870: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145880: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`Shadow byte legend (one shadow byte represents 8 application bytes):`
`  Addressable:           00`
`  Partially addressable: 01 02 03 04 05 06 07 `
`  Heap left redzone:       fa`
`  Heap right redzone:      fb`
`  Freed heap region:       fd`
`  Stack left redzone:      f1`
`  Stack mid redzone:       f2`
`  Stack right redzone:     f3`
`  Stack partial redzone:   f4`
`  Stack after return:      f5`
`  Stack use after scope:   f8`
`  Global redzone:          f9`
`  Global init order:       f6`
`  Poisoned by user:        f7`
`  Container overflow:      fc`
`  Array cookie:            ac`
`  Intra object redzone:    bb`
`  ASan internal:           fe`
`==21864==ABORTING`
### System Configuration
<!-- Tell us about the environment where you are experiencing the bug -->

- ImageMagick version:
Version: ImageMagick 7.0.9-7 Q16 x86_64 2019-11-27 https://imagemagick.org
Copyright: ? 1999-2020 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC HDRI OpenMP(4.0) 
Delegates (built-in): bzlib djvu fftw fontconfig freetype jbig jng jpeg lcms lqr lzma openexr pangocairo png tiff wmf x xml zlib

- Environment (Operating system, version and so on):
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.5 LTS
Release:	16.04
Codename:	xenial
- Additional information:

<!-- Thanks for reporting the issue to ImageMagick! -->

heap-use-after-free in MngInfoDiscardObject of coders/png.c
This is a GitHub Issue
repo:ImageMagick
owner:ImageMagick
Title : heap-use-after-free in MngInfoDiscardObject of coders/png.c
Issue date:

--- start body ---
### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have verified that I am using the latest version of ImageMagick
- [X] I have searched [open](https://github.com/ImageMagick/ImageMagick/issues) and [closed](https://github.com/ImageMagick/ImageMagick/issues?q=is%3Aissue+is%3Aclosed) issues to ensure it has not already been reported

### Description
<!-- A description of the bug or feature -->
There is a heap-use-after-free vulnerability in function MngInfoDiscardObject of coders/png.c whick can be reproduced as below.

### Steps to Reproduce
<!-- List of steps, sample code, failing test or link to a project that reproduces the behavior.
     Make sure you place a stack trace inside a code (```) block to avoid linking unrelated issues -->
[poc](https://drive.google.com/open?id=1z31pFLCb5tluwW_n_l7Dh9C_YWBs_IvX)
magick convert $poc /dev/null
`=================================================================`
`==21864==ERROR: AddressSanitizer: heap-use-after-free on address 0xf0a2c1ad at pc 0x083e0eb3 bp 0xffafc948 sp 0xffafc938`
`READ of size 1 at 0xf0a2c1ad thread T0`
`    #0 0x83e0eb2 in MngInfoDiscardObject coders/png.c:1569`
`    #1 0x83e1237 in MngInfoFreeStruct coders/png.c:1609`
`    #2 0x84021f5 in ReadMNGImage coders/png.c:7790`
`    #3 0x84fccbf in ReadImage MagickCore/constitute.c:553`
`    #4 0x810a116 in ReadStream MagickCore/stream.c:1043`
`    #5 0x84fbcd1 in PingImage MagickCore/constitute.c:273`
`    #6 0x84fc2e3 in PingImages MagickCore/constitute.c:374`
`    #7 0x8991f79 in IdentifyImageCommand MagickWand/identify.c:304`
`    #8 0x8a26ba8 in MagickCommandGenesis MagickWand/mogrify.c:185`
`    #9 0x8053b31 in MagickMain utilities/magick.c:149`
`    #10 0x8053d72 in main utilities/magick.c:180`
`    #11 0xf60c5636 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x18636)`
`    #12 0x80535a0  (/home/ImageMagick/utilities/magick+0x80535a0)`

`0xf0a2c1ad is located 8365 bytes inside of 12368-byte region [0xf0a2a100,0xf0a2d150)`
`freed by thread T0 here:`
`    #0 0xf7241a84 in free (/usr/lib/i386-linux-gnu/libasan.so.2+0x96a84)`
`    #1 0x808a8e4 in RelinquishMagickMemory MagickCore/memory.c:1089`
`    #2 0x83e12de in MngInfoFreeStruct coders/png.c:1614`
`    #3 0x8401702 in ReadOneMNGImage coders/png.c:7681`
`    #4 0x84021e4 in ReadMNGImage coders/png.c:7789`
`    #5 0x84fccbf in ReadImage MagickCore/constitute.c:553`
`    #6 0x810a116 in ReadStream MagickCore/stream.c:1043`
`    #7 0x84fbcd1 in PingImage MagickCore/constitute.c:273`
`    #8 0x84fc2e3 in PingImages MagickCore/constitute.c:374`
`    #9 0x8991f79 in IdentifyImageCommand MagickWand/identify.c:304`
`    #10 0x8a26ba8 in MagickCommandGenesis MagickWand/mogrify.c:185`
`    #11 0x8053b31 in MagickMain utilities/magick.c:149`
`    #12 0x8053d72 in main utilities/magick.c:180`
`    #13 0xf60c5636 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x18636)`

`previously allocated by thread T0 here:`
`    #0 0xf7241dee in malloc (/usr/lib/i386-linux-gnu/libasan.so.2+0x96dee)`
`    #1 0x80899a2 in AcquireMagickMemory MagickCore/memory.c:488`
`    #2 0x8402117 in ReadMNGImage coders/png.c:7780`
`    #3 0x84fccbf in ReadImage MagickCore/constitute.c:553`
`    #4 0x810a116 in ReadStream MagickCore/stream.c:1043`
`    #5 0x84fbcd1 in PingImage MagickCore/constitute.c:273`
`    #6 0x84fc2e3 in PingImages MagickCore/constitute.c:374`
`    #7 0x8991f79 in IdentifyImageCommand MagickWand/identify.c:304`
`    #8 0x8a26ba8 in MagickCommandGenesis MagickWand/mogrify.c:185`
`    #9 0x8053b31 in MagickMain utilities/magick.c:149`
`    #10 0x8053d72 in main utilities/magick.c:180`
`    #11 0xf60c5636 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x18636)`

`SUMMARY: AddressSanitizer: heap-use-after-free coders/png.c:1569 MngInfoDiscardObject`
`Shadow bytes around the buggy address:`
`  0x3e1457e0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e1457f0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145800: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145810: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145820: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`=>0x3e145830: fd fd fd fd fd[fd]fd fd fd fd fd fd fd fd fd fd`
`  0x3e145840: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145850: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145860: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145870: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`  0x3e145880: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd`
`Shadow byte legend (one shadow byte represents 8 application bytes):`
`  Addressable:           00`
`  Partially addressable: 01 02 03 04 05 06 07 `
`  Heap left redzone:       fa`
`  Heap right redzone:      fb`
`  Freed heap region:       fd`
`  Stack left redzone:      f1`
`  Stack mid redzone:       f2`
`  Stack right redzone:     f3`
`  Stack partial redzone:   f4`
`  Stack after return:      f5`
`  Stack use after scope:   f8`
`  Global redzone:          f9`
`  Global init order:       f6`
`  Poisoned by user:        f7`
`  Container overflow:      fc`
`  Array cookie:            ac`
`  Intra object redzone:    bb`
`  ASan internal:           fe`
`==21864==ABORTING`
### System Configuration
<!-- Tell us about the environment where you are experiencing the bug -->

- ImageMagick version:
Version: ImageMagick 7.0.9-7 Q16 x86_64 2019-11-27 https://imagemagick.org
Copyright: ? 1999-2020 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC HDRI OpenMP(4.0) 
Delegates (built-in): bzlib djvu fftw fontconfig freetype jbig jng jpeg lcms lqr lzma openexr pangocairo png tiff wmf x xml zlib

- Environment (Operating system, version and so on):
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.5 LTS
Release:	16.04
Codename:	xenial
- Additional information:

<!-- Thanks for reporting the issue to ImageMagick! -->

--- end body ---
