avformat/subviewer1dec: 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 9751d7515222c7b58d0c6fb31aec6e0464c0f338)

libavformat/subviewer1dec.c

@@ -77,8 +77,10 @@ static int subviewer1_read_header(AVFormatContext *s)
                     sub->duration = pts_start - sub->pts;
             } else {
                 sub = ff_subtitles_queue_insert(&subviewer1->q, line, len, 0);
-                if (!sub)
+                if (!sub) {
+                    ff_subtitles_queue_clean(&subviewer1->q);
                     return AVERROR(ENOMEM);
+                }
                 sub->pos = pos;
                 sub->pts = pts_start;
                 sub->duration = -1;