## Description
There is a divison-by-zero issue caused runtime error and is detected by UndefinedBehaviorSanitizer

## System info
```
Ubuntu 20.04.2 LTS
clang version 12.0.0-++20210402082642+04ba60cfe598-1~exp1~20210402063359.71
MP4Box - GPAC version 1.1.0-DEV-rev1663-g881c6a94a-master
```

## Build command
```
./configure --static-mp4box --prefix=`realpath ./install` --enable-sanitizer --cc=clang --cxx=clang++
```

## crash command
MP4Box -isma -timescale 600 -out /dev/null poc_file

## Pocs
[poc.zip](https://github.com/gpac/gpac/files/7939092/poc.zip)

## Crash output
poc_13
```
[iso file] Unknown box type vmhf in parent minf
Converting to ISMA Audio-Video MP4 file
media_tools/isom_tools.c:558:9: runtime error: division by zero
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior media_tools/isom_tools.c:558:9 in /zhengjie/collect/collec.sh: line 13: 20639 Aborted (core dumped)
```


Division by zero
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Division by zero
Issue date:

--- start body ---
## Description
There is a divison-by-zero issue caused runtime error and is detected by UndefinedBehaviorSanitizer

## System info
```
Ubuntu 20.04.2 LTS
clang version 12.0.0-++20210402082642+04ba60cfe598-1~exp1~20210402063359.71
MP4Box - GPAC version 1.1.0-DEV-rev1663-g881c6a94a-master
```

## Build command
```
./configure --static-mp4box --prefix=`realpath ./install` --enable-sanitizer --cc=clang --cxx=clang++
```

## crash command
MP4Box -isma -timescale 600 -out /dev/null poc_file

## Pocs
[poc.zip](https://github.com/gpac/gpac/files/7939092/poc.zip)

## Crash output
poc_13
```
[iso file] Unknown box type vmhf in parent minf
Converting to ISMA Audio-Video MP4 file
media_tools/isom_tools.c:558:9: runtime error: division by zero
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior media_tools/isom_tools.c:558:9 in /zhengjie/collect/collec.sh: line 13: 20639 Aborted (core dumped)
```


--- end body ---
