matroskadec: reset num_levels on seek.

Otherwise when we run into levels beyond the max. allowed
playback will be permanently broken.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavformat/matroskadec.c

@@ -2067,6 +2067,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
     matroska->skip_to_keyframe = !(flags & AVSEEK_FLAG_ANY);
     matroska->skip_to_timecode = st->index_entries[index].timestamp;
     matroska->done = 0;
+    matroska->num_levels = 0;
     ff_update_cur_dts(s, st, st->index_entries[index].timestamp);
     return 0;
 }