matroska: Clear prev_pkt between seeks.

The new incremental parser doesn't always clear prev_pkt,
however the packet queue is cleared when seeking. Which leads
to a use-after-free.

Verified using Valgrind.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

libavformat/matroskadec.c

@@ -2094,6 +2094,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
         avio_seek(s->pb, st->index_entries[st->nb_index_entries-1].pos, SEEK_SET);
         matroska->current_id = 0;
         while ((index = av_index_search_timestamp(st, timestamp, flags)) < 0) {
+            matroska->prev_pkt = NULL;
             matroska_clear_queue(matroska);
             if (matroska_parse_cluster(matroska) < 0)
                 break;