avformat/subtitles: Don't increment packet counter prematurely

Do it only if the packet has been successfully allocated in
av_new_packet() -- otherwise on error a completely uninitialized packet
would be unreferenced later.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 6bd8bcc2ac)

libavformat/subtitles.c

@@ -132,9 +132,10 @@ AVPacket *ff_subtitles_queue_insert(FFDemuxSubtitlesQueue *q,
         if (!subs)
             return NULL;
         q->subs = subs;
-        sub = &subs[q->nb_subs++];
+        sub = &subs[q->nb_subs];
         if (av_new_packet(sub, len) < 0)
             return NULL;
+        q->nb_subs++;
         sub->flags |= AV_PKT_FLAG_KEY;
         sub->pts = sub->dts = 0;
         memcpy(sub->data, event, len);