matroska: Fix use after free

Signed-off-by: NDale Curtis <dalecurtis@chromium.org>
Signed-off-by: NLuca Barbato <lu_zero@gentoo.org>

libavformat/matroskadec.c

@@ -1744,6 +1744,7 @@ static int matroska_deliver_packet(MatroskaDemuxContext *matroska,
  */
 static void matroska_clear_queue(MatroskaDemuxContext *matroska)
 {
+    matroska->prev_pkt = NULL;
     if (matroska->packets) {
         int n;
         for (n = 0; n < matroska->num_packets; n++) {
@@ -2231,7 +2232,6 @@ 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;