Treat disposition==0 as undefined and don't write the default flag

Originally committed as revision 16651 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskaenc.c

@@ -560,6 +560,7 @@ static int mkv_write_tracks(AVFormatContext *s)
         else
             put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE, "und");
 
+        if (st->disposition)
         put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGDEFAULT, !!(st->disposition & AV_DISPOSITION_DEFAULT));
 
         // look for a codec ID string specific to mkv to use,