In GPAC MP4Box 1.1.0, there is a Null pointer reference in the function gf_filter_pid_get_packet function in src/filter_core/filter_pid.c:5394, as demonstrated by GPAC. This can cause a denial of service (DOS).
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 ...)

Step to reproduce:
```
1.get latest commit code (GPAC version 1.1.0-DEV-rev1170-g592ba2689-master)
2.compile with --enable-sanitizer
3.run  ./gpac nhmlr:reframe=1:gpac:index=1.0:gpac:src=
```


**Im not sure if it's a correct usage of "nhmlr filter" , or by which way could i parse nhml file?**

Env:
Ubunut 20.04 , clang 12.0.1

ASAN report

```
==2311904==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fa16d0fb321 bp 0x7fff730acdb0 sp 0x7fff730acc20 T0)
==2311904==The signal is caused by a READ memory access.
==2311904==Hint: address points to the zero page.
    #0 0x7fa16d0fb321 in gf_filter_pid_get_packet /home/lly/pro/gpac_public/src/filter_core/filter_pid.c:5394:6
    #1 0x7fa16d22468a in nhmldmx_process /home/lly/pro/gpac_public/src/filters/dmx_nhml.c:1320:8
    #2 0x7fa16d15a431 in gf_filter_process_task /home/lly/pro/gpac_public/src/filter_core/filter.c:2441:7
    #3 0x7fa16d13d2a7 in gf_fs_thread_proc /home/lly/pro/gpac_public/src/filter_core/filter_session.c:1640:3
    #4 0x7fa16d13c850 in gf_fs_run /home/lly/pro/gpac_public/src/filter_core/filter_session.c:1877:2
    #5 0x4d12ed in gpac_main /home/lly/pro/gpac_public/applications/gpac/main.c:2254:7
    #6 0x7fa16c0210b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #7 0x41fe0d in _start (/home/lly/pro/gpac_public/bin/gcc/gpac+0x41fe0d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/lly/pro/gpac_public/src/filter_core/filter_pid.c:5394:6 in gf_filter_pid_get_packet
```





Null pointer reference in GPAC at src/filter_core/filter_pid.c:5394
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Null pointer reference in GPAC at src/filter_core/filter_pid.c:5394
Issue date:

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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 ...)

Step to reproduce:
```
1.get latest commit code (GPAC version 1.1.0-DEV-rev1170-g592ba2689-master)
2.compile with --enable-sanitizer
3.run  ./gpac nhmlr:reframe=1:gpac:index=1.0:gpac:src=
```


**Im not sure if it's a correct usage of "nhmlr filter" , or by which way could i parse nhml file?**

Env:
Ubunut 20.04 , clang 12.0.1

ASAN report

```
==2311904==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fa16d0fb321 bp 0x7fff730acdb0 sp 0x7fff730acc20 T0)
==2311904==The signal is caused by a READ memory access.
==2311904==Hint: address points to the zero page.
    #0 0x7fa16d0fb321 in gf_filter_pid_get_packet /home/lly/pro/gpac_public/src/filter_core/filter_pid.c:5394:6
    #1 0x7fa16d22468a in nhmldmx_process /home/lly/pro/gpac_public/src/filters/dmx_nhml.c:1320:8
    #2 0x7fa16d15a431 in gf_filter_process_task /home/lly/pro/gpac_public/src/filter_core/filter.c:2441:7
    #3 0x7fa16d13d2a7 in gf_fs_thread_proc /home/lly/pro/gpac_public/src/filter_core/filter_session.c:1640:3
    #4 0x7fa16d13c850 in gf_fs_run /home/lly/pro/gpac_public/src/filter_core/filter_session.c:1877:2
    #5 0x4d12ed in gpac_main /home/lly/pro/gpac_public/applications/gpac/main.c:2254:7
    #6 0x7fa16c0210b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #7 0x41fe0d in _start (/home/lly/pro/gpac_public/bin/gcc/gpac+0x41fe0d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/lly/pro/gpac_public/src/filter_core/filter_pid.c:5394:6 in gf_filter_pid_get_packet
```





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