avformat/assdec: Fix memleak upon read header failure

The already parsed subtitles (contained in an FFDemuxSubtitlesQueue)
would leak if an error happened upon reading a subsequent subtitle
or if creating the extradata failed.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/assdec.c

@@ -160,6 +160,8 @@ static int ass_read_header(AVFormatContext *s)
     ff_subtitles_queue_finalize(s, &ass->q);
 
 end:
+    if (res < 0)
+        ass_read_close(s);
     av_bprint_finalize(&header, NULL);
     av_bprint_finalize(&line,   NULL);
     av_bprint_finalize(&rline,  NULL);