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 <lq@onvideo.cn>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>