matroskadec: add correct extradata offset for V_MS/VFW/FOURCC tracks

Originally committed as revision 19694 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -1246,6 +1246,7 @@ static int matroska_read_header(AVFormatContext *s, AVFormatParameters *ap)
             && track->codec_priv.data != NULL) {
             track->video.fourcc = AV_RL32(track->codec_priv.data + 16);
             codec_id = ff_codec_get_id(ff_codec_bmp_tags, track->video.fourcc);
+            extradata_offset = 40;
         } else if (!strcmp(track->codec_id, "A_MS/ACM")
                    && track->codec_priv.size >= 18
                    && track->codec_priv.data != NULL) {