avformat/sccdec: 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 f3c63e67bb)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/sccdec.c

@@ -132,7 +132,7 @@ static int scc_read_header(AVFormatContext *s)
 
         sub = ff_subtitles_queue_insert(&scc->q, out, i, 0);
         if (!sub)
-            return AVERROR(ENOMEM);
+            goto fail;
 
         sub->pos = pos;
         sub->pts = ts_start;
@@ -144,6 +144,9 @@ static int scc_read_header(AVFormatContext *s)
     ff_subtitles_queue_finalize(s, &scc->q);
 
     return ret;
+fail:
+    ff_subtitles_queue_clean(&scc->q);
+    return AVERROR(ENOMEM);
 }
 
 static int scc_read_packet(AVFormatContext *s, AVPacket *pkt)