avformat/dashdec: Fix memleaks on error to add representation to dynarray

avformat/dashdec: Fix memleaks on error to add representation to dynarray

Up until now, the DASH demuxer used av_dynarray_add() to add
audio/video/subtitles representations to arrays. Yet av_dynarray_add()
frees the array upon failure, leading to leaks of its elements;
furthermore, the element to be added leaks, too.

This has been fixed by using av_dynarray_add_nofree() instead and by
freeing the elements that could not be added to the list. Furthermore,
errors from this are now checked and returned.

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>