avformat/dashdec: Free subtitle representations on exit

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit eb344862e4)

libavformat/dashdec.c

@@ -2257,6 +2257,7 @@ static int dash_close(AVFormatContext *s)
     DASHContext *c = s->priv_data;
     free_audio_list(c);
     free_video_list(c);
+    free_subtitle_list(c);
     av_dict_free(&c->avio_opts);
     av_freep(&c->base_url);
     return 0;