Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

Version:
```
MP4Box -version 
MP4Box - GPAC version 1.1.0-DEV-rev1574-g8b22f0912-master
(c) 2000-2021 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io

Please cite our work in your research:
        GPAC Filters: https://doi.org/10.1145/3339825.3394929
        GPAC: https://doi.org/10.1145/1291233.1291452

GPAC Configuration: -g --static-mp4box --prefix=/home/hankle/Testing/source/gpac/cmakebuild
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_IPV6 GPAC_HAS_SSL GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_JPEG GPAC_HAS_PNG GPAC_HAS_LINUX_DVB
```
command:

```
./MP4Box -diso -out /dev/null poc
```
[poc.zip](https://github.com/gpac/gpac/files/7791862/poc.zip)

Result
```
Segmentation fault
```

bt
```
Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
RAX: 0xffffffff 
RBX: 0x7db8 
RCX: 0x0 
RDX: 0x5555555e6df0 --> 0xb0e08cb0e08cb0e0 
RSI: 0x7ffff7e04189 (" value=\"%s\">\n")
RDI: 0x5555555decf0 --> 0x7ffffbad2c84 
RBP: 0x1 
RSP: 0x7fffffff7ff0 --> 0x5555555db650 --> 0x73747473 ('stts')
RIP: 0x7ffff78f7da1 (<xtra_box_dump+177>:       mov    BYTE PTR [r14+rax*1],0x0)
R8 : 0x3 
R9 : 0x5555555e4b1a --> 0x80000590900ddc7 
R10: 0x5555555e9a16 --> 0x0 
R11: 0x5c7 
R12: 0x5555555decf0 --> 0x7ffffbad2c84 
R13: 0x5555555e2990 --> 0x58747261 ('artX')
R14: 0x5555555e6df0 --> 0xb0e08cb0e08cb0e0 
R15: 0x7ffff7e71725 --> 0x2020200058323025 ('%02X')
EFLAGS: 0x10202 (carry parity adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x7ffff78f7d95 <xtra_box_dump+165>:  lea    rsi,[rip+0x50c3ed]        # 0x7ffff7e04189
   0x7ffff78f7d9c <xtra_box_dump+172>:  mov    rdi,r12
   0x7ffff78f7d9f <xtra_box_dump+175>:  mov    eax,eax
=> 0x7ffff78f7da1 <xtra_box_dump+177>:  mov    BYTE PTR [r14+rax*1],0x0
   0x7ffff78f7da6 <xtra_box_dump+182>:  xor    eax,eax
   0x7ffff78f7da8 <xtra_box_dump+184>:  call   0x7ffff77696d0 <gf_fprintf@plt>
   0x7ffff78f7dad <xtra_box_dump+189>:  mov    rdi,r14
   0x7ffff78f7db0 <xtra_box_dump+192>:  call   0x7ffff7767fc0 <gf_free@plt>
[------------------------------------stack-------------------------------------]
0000| 0x7fffffff7ff0 --> 0x5555555db650 --> 0x73747473 ('stts')
0008| 0x7fffffff7ff8 --> 0x1 
0016| 0x7fffffff8000 --> 0x5555555e4b1a --> 0x80000590900ddc7 
0024| 0x7fffffff8008 --> 0xdb67785d868c8800 
0032| 0x7fffffff8010 --> 0x5555555decf0 --> 0x7ffffbad2c84 
0040| 0x7fffffff8018 --> 0x5555555da950 --> 0x0 
0048| 0x7fffffff8020 --> 0x5555555e2990 --> 0x58747261 ('artX')
0056| 0x7fffffff8028 --> 0x5555555decf0 --> 0x7ffffbad2c84 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x00007ffff78f7da1 in xtra_box_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
gdb-peda$ bt
#0  0x00007ffff78f7da1 in xtra_box_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
#1  0x00007ffff78fa5f2 in gf_isom_box_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
#2  0x00007ffff78e99f6 in gf_isom_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
#3  0x0000555555588c15 in dump_isom_xml ()
#4  0x000055555557c564 in mp4boxMain ()
#5  0x00007ffff74dc0b3 in __libc_start_main (main=0x55555556d420 <main>, argc=0x5, argv=0x7fffffffe318, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe308)
    at ../csu/libc-start.c:308
#6  0x000055555556d45e in _start ()
```


Untrusted pointer dereference in xtra_box_dump ()
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Untrusted pointer dereference in xtra_box_dump ()
Issue date:

--- start body ---
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

Version:
```
MP4Box -version 
MP4Box - GPAC version 1.1.0-DEV-rev1574-g8b22f0912-master
(c) 2000-2021 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io

Please cite our work in your research:
        GPAC Filters: https://doi.org/10.1145/3339825.3394929
        GPAC: https://doi.org/10.1145/1291233.1291452

GPAC Configuration: -g --static-mp4box --prefix=/home/hankle/Testing/source/gpac/cmakebuild
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_IPV6 GPAC_HAS_SSL GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_JPEG GPAC_HAS_PNG GPAC_HAS_LINUX_DVB
```
command:

```
./MP4Box -diso -out /dev/null poc
```
[poc.zip](https://github.com/gpac/gpac/files/7791862/poc.zip)

Result
```
Segmentation fault
```

bt
```
Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
RAX: 0xffffffff 
RBX: 0x7db8 
RCX: 0x0 
RDX: 0x5555555e6df0 --> 0xb0e08cb0e08cb0e0 
RSI: 0x7ffff7e04189 (" value=\"%s\">\n")
RDI: 0x5555555decf0 --> 0x7ffffbad2c84 
RBP: 0x1 
RSP: 0x7fffffff7ff0 --> 0x5555555db650 --> 0x73747473 ('stts')
RIP: 0x7ffff78f7da1 (<xtra_box_dump+177>:       mov    BYTE PTR [r14+rax*1],0x0)
R8 : 0x3 
R9 : 0x5555555e4b1a --> 0x80000590900ddc7 
R10: 0x5555555e9a16 --> 0x0 
R11: 0x5c7 
R12: 0x5555555decf0 --> 0x7ffffbad2c84 
R13: 0x5555555e2990 --> 0x58747261 ('artX')
R14: 0x5555555e6df0 --> 0xb0e08cb0e08cb0e0 
R15: 0x7ffff7e71725 --> 0x2020200058323025 ('%02X')
EFLAGS: 0x10202 (carry parity adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x7ffff78f7d95 <xtra_box_dump+165>:  lea    rsi,[rip+0x50c3ed]        # 0x7ffff7e04189
   0x7ffff78f7d9c <xtra_box_dump+172>:  mov    rdi,r12
   0x7ffff78f7d9f <xtra_box_dump+175>:  mov    eax,eax
=> 0x7ffff78f7da1 <xtra_box_dump+177>:  mov    BYTE PTR [r14+rax*1],0x0
   0x7ffff78f7da6 <xtra_box_dump+182>:  xor    eax,eax
   0x7ffff78f7da8 <xtra_box_dump+184>:  call   0x7ffff77696d0 <gf_fprintf@plt>
   0x7ffff78f7dad <xtra_box_dump+189>:  mov    rdi,r14
   0x7ffff78f7db0 <xtra_box_dump+192>:  call   0x7ffff7767fc0 <gf_free@plt>
[------------------------------------stack-------------------------------------]
0000| 0x7fffffff7ff0 --> 0x5555555db650 --> 0x73747473 ('stts')
0008| 0x7fffffff7ff8 --> 0x1 
0016| 0x7fffffff8000 --> 0x5555555e4b1a --> 0x80000590900ddc7 
0024| 0x7fffffff8008 --> 0xdb67785d868c8800 
0032| 0x7fffffff8010 --> 0x5555555decf0 --> 0x7ffffbad2c84 
0040| 0x7fffffff8018 --> 0x5555555da950 --> 0x0 
0048| 0x7fffffff8020 --> 0x5555555e2990 --> 0x58747261 ('artX')
0056| 0x7fffffff8028 --> 0x5555555decf0 --> 0x7ffffbad2c84 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x00007ffff78f7da1 in xtra_box_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
gdb-peda$ bt
#0  0x00007ffff78f7da1 in xtra_box_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
#1  0x00007ffff78fa5f2 in gf_isom_box_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
#2  0x00007ffff78e99f6 in gf_isom_dump () from /home/hankle/Testing/gpac/bin/gcc/libgpac.so.10
#3  0x0000555555588c15 in dump_isom_xml ()
#4  0x000055555557c564 in mp4boxMain ()
#5  0x00007ffff74dc0b3 in __libc_start_main (main=0x55555556d420 <main>, argc=0x5, argv=0x7fffffffe318, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe308)
    at ../csu/libc-start.c:308
#6  0x000055555556d45e in _start ()
```


--- end body ---
