matroskadec: set cur_dts after seek

patch by elupus   elupus _at_ ecce _dot_ se

Originally committed as revision 13748 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -3177,6 +3177,7 @@ matroska_read_seek (AVFormatContext *s, int stream_index, int64_t timestamp,
     matroska->skip_to_keyframe = !(flags & AVSEEK_FLAG_ANY);
     matroska->skip_to_stream = st;
     matroska->peek_id = 0;
+    av_update_cur_dts(s, st, st->index_entries[index].timestamp);
     return 0;
 }