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 <[email protected]>
diff --git a/libavformat/mpsubdec.c b/libavformat/mpsubdec.c
index 82c7345..e7b83a1 100644
--- a/libavformat/mpsubdec.c
+++ b/libavformat/mpsubdec.c
@@ -154,8 +154,10 @@
     }
 
     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;