avformat/subviewerdec: 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.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit a708f65273)

libavformat/subviewerdec.c

@@ -172,6 +172,8 @@ static int subviewer_read_header(AVFormatContext *s)
     ff_subtitles_queue_finalize(s, &subviewer->q);
 
 end:
+    if (res < 0)
+        ff_subtitles_queue_clean(&subviewer->q);
     av_bprint_finalize(&header, NULL);
     return res;
 }