A Null Pointer Dereference vulnerability exitgs in GPAC 1.0.1 in MP4Box via __strlen_avx2, which causes a Denial of Service.
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [Yes ] I looked for a similar issue and couldn't find any.
- [ Yes] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [ Yes] 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-rev1527-g6fcf9819e-master
(c) 2000-2021 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
 MINI build (encoders, decoders, audio and video output disabled)

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: --static-mp4box --prefix=/home/zxq/CVE_testing/sourceproject/gpac/cmakebuild --enable-debug
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_FREETYPE GPAC_HAS_JPEG GPAC_HAS_PNG  GPAC_DISABLE_3D 
```
**System information**
Ubuntu 20.04.1 LTS, gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)

**command:**
```
./bin/gcc/MP4Box -bt POC1
```


[POC1.zip](https://github.com/gpac/gpac/files/7711140/POC1.zip)


**Result**
```
[5]    2204206 segmentation fault  ./sourceproject/momey/gpac/bin/gcc/MP4Box -bt 
```
**Gdb information**
```
Stopped reason: SIGSEGV
__strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:65
65      ../sysdeps/x86_64/multiarch/strlen-avx2.S: No such file or directory.
gdb-peda$ bt
#0  __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:65
#1  0x00007ffff755a503 in __GI___strdup (s=0x0) at strdup.c:41
#2  0x00007ffff7851545 in gf_svg_dump_attribute () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#3  0x00007ffff7a497e2 in gf_dump_svg_element () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#4  0x00007ffff7a4a9b0 in gf_sm_dump_command_list () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#5  0x00007ffff7a5173d in gf_sm_dump () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#6  0x0000555555585418 in dump_isom_scene ()
#7  0x000055555557c42c in mp4boxMain ()
#8  0x00007ffff74df0b3 in __libc_start_main (main=0x55555556d420 <main>, argc=0x3, argv=0x7fffffffe248, init=<optimized out>, 
    fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe238) at ../csu/libc-start.c:308
#9  0x000055555556d45e in _start ()


```

Null Pointer Dereference in __strlen_avx2 ()
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Null Pointer Dereference in __strlen_avx2 ()
Issue date:

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

- [Yes ] I looked for a similar issue and couldn't find any.
- [ Yes] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [ Yes] 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-rev1527-g6fcf9819e-master
(c) 2000-2021 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
 MINI build (encoders, decoders, audio and video output disabled)

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: --static-mp4box --prefix=/home/zxq/CVE_testing/sourceproject/gpac/cmakebuild --enable-debug
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_FREETYPE GPAC_HAS_JPEG GPAC_HAS_PNG  GPAC_DISABLE_3D 
```
**System information**
Ubuntu 20.04.1 LTS, gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)

**command:**
```
./bin/gcc/MP4Box -bt POC1
```


[POC1.zip](https://github.com/gpac/gpac/files/7711140/POC1.zip)


**Result**
```
[5]    2204206 segmentation fault  ./sourceproject/momey/gpac/bin/gcc/MP4Box -bt 
```
**Gdb information**
```
Stopped reason: SIGSEGV
__strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:65
65      ../sysdeps/x86_64/multiarch/strlen-avx2.S: No such file or directory.
gdb-peda$ bt
#0  __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:65
#1  0x00007ffff755a503 in __GI___strdup (s=0x0) at strdup.c:41
#2  0x00007ffff7851545 in gf_svg_dump_attribute () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#3  0x00007ffff7a497e2 in gf_dump_svg_element () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#4  0x00007ffff7a4a9b0 in gf_sm_dump_command_list () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#5  0x00007ffff7a5173d in gf_sm_dump () from /home/zxq/CVE_testing/sourceproject/momey/gpac/bin/gcc/libgpac.so.10
#6  0x0000555555585418 in dump_isom_scene ()
#7  0x000055555557c42c in mp4boxMain ()
#8  0x00007ffff74df0b3 in __libc_start_main (main=0x55555556d420 <main>, argc=0x3, argv=0x7fffffffe248, init=<optimized out>, 
    fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe238) at ../csu/libc-start.c:308
#9  0x000055555556d45e in _start ()


```

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