avformat/dashdec: Fix memleak when resolve_content_path

avformat/dashdec: Fix memleak when resolve_content_path Can be reproduced with: valgrind --leak-check=full -v ffmpeg -i
http://yt-dash-mse-test.commondatastorage.googleapis.com/media/motion-20120802-manifest.mpd Reviewed-by: Steven Liu 
Signed-off-by: Steven Liu 

• [DH] libavformat/dashdec.c