avformat/mpsubdec: Fix memleak upon read header failure

The already parsed subtitles (contained in an FFDemuxSubtitlesQueue)
would leak if an error happened upon creating an AVStream.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit a5ed8aeea4f4199e89520c3fdbd9d07ae7fc3c3f)

libavformat/mpsubdec.c

@@ -154,8 +154,10 @@ static int mpsub_read_header(AVFormatContext *s)
     }
 
     st = avformat_new_stream(s, NULL);
-    if (!st)
-        return AVERROR(ENOMEM);
+    if (!st) {
+        res = AVERROR(ENOMEM);
+        goto end;
+    }
     avpriv_set_pts_info(st, 64, pts_info.den, pts_info.num);
     st->codecpar->codec_type = AVMEDIA_TYPE_SUBTITLE;
     st->codecpar->codec_id   = AV_CODEC_ID_TEXT;