Potential memory leak in [ff_mx.c](https://github.com/gpac/gpac/blob/master/src/filters/ff_mx.c#L373) line 373. Calling av_dict_copy() without calling av_dict_free() to free the memory will cause a memory leak.[Doc](https://ffmpeg.org/doxygen/trunk/group__lavu__dict.html#ga1bafd682b1fbb90e48a4cc3814b820f7) says "av_dict_free() Free all the memory allocated for an [AVDictionary](https://ffmpeg.org/doxygen/trunk/structAVDictionary.html) struct and all keys and values."
![image](https://user-images.githubusercontent.com/39425646/214762607-08c18e82-4fa0-4992-81ac-93b238b67736.png)

Potential memory leak in src/filters/ff_mx.c
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Potential memory leak in src/filters/ff_mx.c
Issue date:

--- start body ---
Potential memory leak in [ff_mx.c](https://github.com/gpac/gpac/blob/master/src/filters/ff_mx.c#L373) line 373. Calling av_dict_copy() without calling av_dict_free() to free the memory will cause a memory leak.[Doc](https://ffmpeg.org/doxygen/trunk/group__lavu__dict.html#ga1bafd682b1fbb90e48a4cc3814b820f7) says "av_dict_free() Free all the memory allocated for an [AVDictionary](https://ffmpeg.org/doxygen/trunk/structAVDictionary.html) struct and all keys and values."
![image](https://user-images.githubusercontent.com/39425646/214762607-08c18e82-4fa0-4992-81ac-93b238b67736.png)

--- end body ---
