Hello,
There is a Null Pointer Dereference in function DumpTrackInfo of filedump.c:2962,this can reproduce on the lattest commit.

System info：
Ubuntu 20.04.1 : clang 10.0.0 , gcc 9.3.0




[poc (2).zip](https://github.com/gpac/gpac/files/6771032/poc.2.zip)






Verification steps：
1.Get the source code of gpac
2.Compile 

```
cd gpac-master
CC=gcc CXX=g++ CFLAGS="-fsanitize=address" CXXFLAGS="-fsanitize=address" ./configure
make
``` 
3.run MP4Box

```
./MP4Box -info poc.mp4
```

command line

```
...
# Track 1 Info - ID 1 - TimeScale 44100
Media Duration 00:00:00.339 
Media Info: Language "Undetermined (und)" - Type "soun:mp4a" - 13 samples
Segmentation fault 


```

GDB info

![null](https://user-images.githubusercontent.com/83855894/124626392-3550d680-deb1-11eb-9bf3-41d0744d5e3b.png)



ASAN info
```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==4002438==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x55d13e590cfb bp 0x60b0000003b0 sp 0x7ffdd8b0e760 T0)
==4002438==The signal is caused by a READ memory access.
==4002438==Hint: address points to the zero page.
    #0 0x55d13e590cfa in DumpTrackInfo /home/.../gpac/gpac-master/applications/mp4box/filedump.c:2962
    #1 0x55d13e59ff44 in DumpMovieInfo /home/.../gpac/gpac-master/applications/mp4box/filedump.c:3777
    #2 0x55d13e55c71d in mp4boxMain /home/.../gpac/gpac-master/applications/mp4box/main.c:5991
    #3 0x7f0ca4b400b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #4 0x55d13e4fef1d in _start (/home/.../gpac/gpac-master/bin/gcc/MP4Box+0x48f1d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/.../gpac/gpac-master/applications/mp4box/filedump.c:2962 in DumpTrackInfo
==4002438==ABORTING
```

source code
```
2960	GF_ISOSample *samp = gf_isom_get_sample(file, trackNum, 1, &oti);
2961	if (samp) {
2962		u32 mhdr = GF_4CC((u8)samp->data[0], (u8)samp->data[1], (u8)samp->data[2], (u8)samp->data[3]);
2963		if (full_dump) fprintf(stderr, "\t");
2964		fprintf(stderr, "%s Audio - %d Channel(s) - SampleRate %d - Layer %d\n",
2965							   gf_mp3_version_name(mhdr),
2966							   gf_mp3_num_channels(mhdr),
2967							   gf_mp3_sampling_rate(mhdr),
2968							   gf_mp3_layer(mhdr)
2969							  );
```





 
A Null Pointer Dereference In DumpTrackInfo
This is a GitHub Issue
repo:gpac
owner:gpac
Title : A Null Pointer Dereference In DumpTrackInfo
Issue date:

--- start body ---
Hello,
There is a Null Pointer Dereference in function DumpTrackInfo of filedump.c:2962,this can reproduce on the lattest commit.

System info：
Ubuntu 20.04.1 : clang 10.0.0 , gcc 9.3.0




[poc (2).zip](https://github.com/gpac/gpac/files/6771032/poc.2.zip)






Verification steps：
1.Get the source code of gpac
2.Compile 

```
cd gpac-master
CC=gcc CXX=g++ CFLAGS="-fsanitize=address" CXXFLAGS="-fsanitize=address" ./configure
make
``` 
3.run MP4Box

```
./MP4Box -info poc.mp4
```

command line

```
...
# Track 1 Info - ID 1 - TimeScale 44100
Media Duration 00:00:00.339 
Media Info: Language "Undetermined (und)" - Type "soun:mp4a" - 13 samples
Segmentation fault 


```

GDB info

![null](https://user-images.githubusercontent.com/83855894/124626392-3550d680-deb1-11eb-9bf3-41d0744d5e3b.png)



ASAN info
```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==4002438==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x55d13e590cfb bp 0x60b0000003b0 sp 0x7ffdd8b0e760 T0)
==4002438==The signal is caused by a READ memory access.
==4002438==Hint: address points to the zero page.
    #0 0x55d13e590cfa in DumpTrackInfo /home/.../gpac/gpac-master/applications/mp4box/filedump.c:2962
    #1 0x55d13e59ff44 in DumpMovieInfo /home/.../gpac/gpac-master/applications/mp4box/filedump.c:3777
    #2 0x55d13e55c71d in mp4boxMain /home/.../gpac/gpac-master/applications/mp4box/main.c:5991
    #3 0x7f0ca4b400b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #4 0x55d13e4fef1d in _start (/home/.../gpac/gpac-master/bin/gcc/MP4Box+0x48f1d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/.../gpac/gpac-master/applications/mp4box/filedump.c:2962 in DumpTrackInfo
==4002438==ABORTING
```

source code
```
2960	GF_ISOSample *samp = gf_isom_get_sample(file, trackNum, 1, &oti);
2961	if (samp) {
2962		u32 mhdr = GF_4CC((u8)samp->data[0], (u8)samp->data[1], (u8)samp->data[2], (u8)samp->data[3]);
2963		if (full_dump) fprintf(stderr, "\t");
2964		fprintf(stderr, "%s Audio - %d Channel(s) - SampleRate %d - Layer %d\n",
2965							   gf_mp3_version_name(mhdr),
2966							   gf_mp3_num_channels(mhdr),
2967							   gf_mp3_sampling_rate(mhdr),
2968							   gf_mp3_layer(mhdr)
2969							  );
```





 
--- end body ---
