ImageMagick 7.0.8-54 Q16 allows Division by Zero in RemoveDuplicateLayers in MagickCore/layer.c.
### 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's a division by zero at MagickCore/layer.c:1616:30

### 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 -->
run following cmd:
`magick "-seed" "0" "-delay" "34<" "(" "magick:rose" "+repage" ")" "(" "magick:rose" "+repage" ")" "-encoding" "Symbol" "-layers" "remove-dups" "-quiet" "tmp"`
 
This is triggered at `time = curr->delay*1000/curr->ticks_per_second;` due to curr->ticks_per_second. 
```
1605   for (; (next=GetNextImageInList(curr)) != (Image *) NULL; curr=next)
1606   { 
1607     if ( curr->columns != next->columns || curr->rows != next->rows
1608          || curr->page.x != next->page.x || curr->page.y != next->page.y )
1609       continue;
1610     bounds=CompareImagesBounds(curr,next,CompareAnyLayer,exception);
1611     if ( bounds.x < 0 ) {
1612       /*
1613         the two images are the same, merge time delays and delete one.
1614       */
1615       size_t time;
1616       time = curr->delay*1000/curr->ticks_per_second;
1617       time += next->delay*1000/next->ticks_per_second;
1618       next->ticks_per_second = 100L;
1619       next->delay = time*curr->ticks_per_second/1000;
1620       next->iterations = curr->iterations;
1621       *images = curr;
1622       (void) DeleteImageFromList(images);
1623     }
1624   }

```

Here's USAN log.
```
MagickCore/layer.c:1616:30: runtime error: division by zero
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior MagickCore/layer.c:1616:30 in 
AddressSanitizer:DEADLYSIGNAL
=================================================================
==28916==ERROR: AddressSanitizer: UNKNOWN SIGNAL on unknown address 0x000000000000 (pc 0x7fe11baee9e7 bp 0x7ffc60e12250 sp 0x7ffc60e11ec0 T0)
    #0 0x7fe11baee9e6 in RemoveDuplicateLayers MagickCore/layer.c:1616:30
    #1 0x7fe11a2600a8 in CLIListOperatorImages MagickWand/operation.c:4266:15
    #2 0x7fe11a273102 in CLIOption MagickWand/operation.c:5308:14
    #3 0x7fe119c6aef4 in ProcessCommandOptions MagickWand/magick-cli.c:477:7
    #4 0x7fe119c6dc54 in MagickImageCommand MagickWand/magick-cli.c:796:5
    #5 0x7fe119d1400e in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #6 0x527976 in MagickMain utilities/magick.c:149:10
    #7 0x5268e1 in main utilities/magick.c:180:10
    #8 0x7fe113884b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #9 0x41b069 in _start (install/bin/magick+0x41b069)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: UNKNOWN SIGNAL MagickCore/layer.c:1616:30 in RemoveDuplicateLayers
==28916==ABORTING

```

### System Configuration
<!-- Tell us about the environment where you are experiencing the bug -->
- ImageMagick version:
Version: ImageMagick 7.0.8-54 Q16 x86_64 2019-07-07 https://imagemagick.org

- Environment (Operating system, version and so on):
Description:	Ubuntu 18.04.1 LTS
Release:	18.04
Codename: bionic

- Additional information:
CC=clang-7 CXX=clang++-7 CFLAGS="-fsanitize=address,undefined -g" CXXFLAGS="-fsanitize=address,undefined -g" ./configure --disable-openmp --without-png

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

Division by Zero at MagickCore/layer.c:1616
This is a GitHub Issue
repo:ImageMagick
owner:ImageMagick
Title : Division by Zero at MagickCore/layer.c:1616
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's a division by zero at MagickCore/layer.c:1616:30

### 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 -->
run following cmd:
`magick "-seed" "0" "-delay" "34<" "(" "magick:rose" "+repage" ")" "(" "magick:rose" "+repage" ")" "-encoding" "Symbol" "-layers" "remove-dups" "-quiet" "tmp"`
 
This is triggered at `time = curr->delay*1000/curr->ticks_per_second;` due to curr->ticks_per_second. 
```
1605   for (; (next=GetNextImageInList(curr)) != (Image *) NULL; curr=next)
1606   { 
1607     if ( curr->columns != next->columns || curr->rows != next->rows
1608          || curr->page.x != next->page.x || curr->page.y != next->page.y )
1609       continue;
1610     bounds=CompareImagesBounds(curr,next,CompareAnyLayer,exception);
1611     if ( bounds.x < 0 ) {
1612       /*
1613         the two images are the same, merge time delays and delete one.
1614       */
1615       size_t time;
1616       time = curr->delay*1000/curr->ticks_per_second;
1617       time += next->delay*1000/next->ticks_per_second;
1618       next->ticks_per_second = 100L;
1619       next->delay = time*curr->ticks_per_second/1000;
1620       next->iterations = curr->iterations;
1621       *images = curr;
1622       (void) DeleteImageFromList(images);
1623     }
1624   }

```

Here's USAN log.
```
MagickCore/layer.c:1616:30: runtime error: division by zero
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior MagickCore/layer.c:1616:30 in 
AddressSanitizer:DEADLYSIGNAL
=================================================================
==28916==ERROR: AddressSanitizer: UNKNOWN SIGNAL on unknown address 0x000000000000 (pc 0x7fe11baee9e7 bp 0x7ffc60e12250 sp 0x7ffc60e11ec0 T0)
    #0 0x7fe11baee9e6 in RemoveDuplicateLayers MagickCore/layer.c:1616:30
    #1 0x7fe11a2600a8 in CLIListOperatorImages MagickWand/operation.c:4266:15
    #2 0x7fe11a273102 in CLIOption MagickWand/operation.c:5308:14
    #3 0x7fe119c6aef4 in ProcessCommandOptions MagickWand/magick-cli.c:477:7
    #4 0x7fe119c6dc54 in MagickImageCommand MagickWand/magick-cli.c:796:5
    #5 0x7fe119d1400e in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #6 0x527976 in MagickMain utilities/magick.c:149:10
    #7 0x5268e1 in main utilities/magick.c:180:10
    #8 0x7fe113884b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #9 0x41b069 in _start (install/bin/magick+0x41b069)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: UNKNOWN SIGNAL MagickCore/layer.c:1616:30 in RemoveDuplicateLayers
==28916==ABORTING

```

### System Configuration
<!-- Tell us about the environment where you are experiencing the bug -->
- ImageMagick version:
Version: ImageMagick 7.0.8-54 Q16 x86_64 2019-07-07 https://imagemagick.org

- Environment (Operating system, version and so on):
Description:	Ubuntu 18.04.1 LTS
Release:	18.04
Codename: bionic

- Additional information:
CC=clang-7 CXX=clang++-7 CFLAGS="-fsanitize=address,undefined -g" CXXFLAGS="-fsanitize=address,undefined -g" ./configure --disable-openmp --without-png

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

--- end body ---
