avformat/webvttdec: 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 c784fe8b86)

libavformat/webvttdec.c

@@ -164,6 +164,8 @@ static int webvtt_read_header(AVFormatContext *s)
     ff_subtitles_queue_finalize(s, &webvtt->q);
 
 end:
+    if (res < 0)
+        ff_subtitles_queue_clean(&webvtt->q);
     av_bprint_finalize(&cue,    NULL);
     return res;
 }