matroskadec: sanitize track time_scale before using it

fix issue628

Originally committed as revision 15280 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -1274,6 +1274,8 @@ static int matroska_read_header(AVFormatContext *s, AVFormatParameters *ap)
             av_log(matroska->ctx, AV_LOG_INFO,
                    "Unknown/unsupported CodecID %s.\n", track->codec_id);
 
+        if (track->time_scale < 0.01)
+            track->time_scale = 1.0;
         av_set_pts_info(st, 64, matroska->time_scale*track->time_scale, 1000*1000*1000); /* 64 bit pts in ns */
 
         st->codec->codec_id = codec_id;