avformat/srtdec: 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 c70409957c)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/srtdec.c

@@ -207,6 +207,8 @@ static int srt_read_header(AVFormatContext *s)
     ff_subtitles_queue_finalize(s, &srt->q);
 
 end:
+    if (res < 0)
+        ff_subtitles_queue_clean(&srt->q);
     av_bprint_finalize(&buf, NULL);
     return res;
 }