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

OS: Linux 5.4.0-1104-azure #&#x2060;110~18.04.1-Ubuntu 64 Bits
Command Line: MP4Box -dash 10000 -out output.mpd input.mkv
App: MP4Box - GPAC version 2.3-DEV-rev47-gfccf4a645-master

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/
Tags: MP4Box, DASH

Im trying to mux MPEG Dash with MP4Box and i got `Segmentation fault (core dumped)` no error message.

Verbose
```
[DASH] Instantiating dasher filter for dst output.mpd with args segdur=10000/1000:profile=full:!sap:buf=1500:!check_dur:pssh=v:subs_sidx=0
[DASH] Instantiating dasher source input.mkv with args smode=splitx:tkid=video:#Representation=1
[DASH] Instantiating dasher source input.mkv with args smode=splitx:tkid=audio:#Representation=2
Probing matroska,webm score:100 size:999968
[FFDmx] opening file input.mkv - av_in 00000000
[NULL] Opening 'input.mkv' for reading
[file] Setting default whitelist 'file,crypto,data'
Probing h263 score:25 size:2048
Probing matroska,webm score:100 size:2048
[matroska,webm] Format matroska,webm probed with size=2048 and score=100
st:0 removing common factor 1000000 from timebase
st:1 removing common factor 1000000 from timebase
[matroska,webm] Before avformat_find_stream_info() pos: 2120 bytes read:32768 seeks:0 nb_streams:2
[matroska,webm] All info found
[matroska,webm] stream 0: start_time: 0 duration: NOPTS
[matroska,webm] stream 1: start_time: 0 duration: NOPTS
[matroska,webm] format: start_time: 0 duration: 8163.75 (estimate from stream) bitrate=2162 kb/s
[matroska,webm] After avformat_find_stream_info() pos: 6304 bytes read:32768 seeks:0 frames:5
[FFDmx] file input.mkv opened - 2 streams
Segmentation fault (core dumped)
```

I can confirm that this issue only occur when using mkv as input file.
Segmentation fault (core dumped)
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Segmentation fault (core dumped)
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

OS: Linux 5.4.0-1104-azure #&#x2060;110~18.04.1-Ubuntu 64 Bits
Command Line: MP4Box -dash 10000 -out output.mpd input.mkv
App: MP4Box - GPAC version 2.3-DEV-rev47-gfccf4a645-master

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/
Tags: MP4Box, DASH

Im trying to mux MPEG Dash with MP4Box and i got `Segmentation fault (core dumped)` no error message.

Verbose
```
[DASH] Instantiating dasher filter for dst output.mpd with args segdur=10000/1000:profile=full:!sap:buf=1500:!check_dur:pssh=v:subs_sidx=0
[DASH] Instantiating dasher source input.mkv with args smode=splitx:tkid=video:#Representation=1
[DASH] Instantiating dasher source input.mkv with args smode=splitx:tkid=audio:#Representation=2
Probing matroska,webm score:100 size:999968
[FFDmx] opening file input.mkv - av_in 00000000
[NULL] Opening 'input.mkv' for reading
[file] Setting default whitelist 'file,crypto,data'
Probing h263 score:25 size:2048
Probing matroska,webm score:100 size:2048
[matroska,webm] Format matroska,webm probed with size=2048 and score=100
st:0 removing common factor 1000000 from timebase
st:1 removing common factor 1000000 from timebase
[matroska,webm] Before avformat_find_stream_info() pos: 2120 bytes read:32768 seeks:0 nb_streams:2
[matroska,webm] All info found
[matroska,webm] stream 0: start_time: 0 duration: NOPTS
[matroska,webm] stream 1: start_time: 0 duration: NOPTS
[matroska,webm] format: start_time: 0 duration: 8163.75 (estimate from stream) bitrate=2162 kb/s
[matroska,webm] After avformat_find_stream_info() pos: 6304 bytes read:32768 seeks:0 frames:5
[FFDmx] file input.mkv opened - 2 streams
Segmentation fault (core dumped)
```

I can confirm that this issue only occur when using mkv as input file.
--- end body ---
