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 https://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

Detailed guidelines: https://gpac.io/bug-reporting/

```
/*
opens an isoMedia File. If fileName is NULL data will be written in memory ;   
write with gf_isom_write() ; use gf_isom_get_bs() to get the data ;   
use gf_isom_delete() to delete the internal data.  
*/
file_ptr = gf_isom_open(NULL, GF_ISOM_OPEN_WRITE, NULL);
```
I make NULL filename with GF_ISOFile for data writing in memory.  
when I try make video progressive mp4, ```gf_isom_add_sample``` Segmentation Fault Raised.  
```
GF_ISOFile *file_ptr = NULL;
// First Arg = NULL data willbe written in memory;
file_ptr = gf_isom_open(NULL, GF_ISOM_OPEN_WRITE, NULL);
gf_isom_set_brand_info
gf_isom_set_timescale
gf_isom_new_track
gf_isom_set_track_enabled
gf_odf_avc_cfg_new
gf_isom_avc_config_new
gf_isom_set_visual_info
gf_isom_avc_config_update
gf_isom_add_sample
```
If there is File name like ```file_ptr = gf_isom_open("output.mp4", GF_ISOM_OPEN_WRITE, NULL);```  
There is no error.     
What's the difference?   

Error is Raised in ```FlushCaptureMode``` in ```gf_isom_add_sample``` Function.    
Error Point  
https://github.com/gpac/gpac/blob/8684dfbcca4163494017e38f8e132751f12bde26/src/isomedia/isom_write.c#L67  

If first argument of strcmp is NULL 
```strcmp``` raise Segmantation Fault

Question About making Progressive mp4
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Question About making Progressive mp4
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 https://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

Detailed guidelines: https://gpac.io/bug-reporting/

```
/*
opens an isoMedia File. If fileName is NULL data will be written in memory ;   
write with gf_isom_write() ; use gf_isom_get_bs() to get the data ;   
use gf_isom_delete() to delete the internal data.  
*/
file_ptr = gf_isom_open(NULL, GF_ISOM_OPEN_WRITE, NULL);
```
I make NULL filename with GF_ISOFile for data writing in memory.  
when I try make video progressive mp4, ```gf_isom_add_sample``` Segmentation Fault Raised.  
```
GF_ISOFile *file_ptr = NULL;
// First Arg = NULL data willbe written in memory;
file_ptr = gf_isom_open(NULL, GF_ISOM_OPEN_WRITE, NULL);
gf_isom_set_brand_info
gf_isom_set_timescale
gf_isom_new_track
gf_isom_set_track_enabled
gf_odf_avc_cfg_new
gf_isom_avc_config_new
gf_isom_set_visual_info
gf_isom_avc_config_update
gf_isom_add_sample
```
If there is File name like ```file_ptr = gf_isom_open("output.mp4", GF_ISOM_OPEN_WRITE, NULL);```  
There is no error.     
What's the difference?   

Error is Raised in ```FlushCaptureMode``` in ```gf_isom_add_sample``` Function.    
Error Point  
https://github.com/gpac/gpac/blob/8684dfbcca4163494017e38f8e132751f12bde26/src/isomedia/isom_write.c#L67  

If first argument of strcmp is NULL 
```strcmp``` raise Segmantation Fault

--- end body ---
