Initial patch by Hannes Domani with the comments:
"Hello

 this fixes the problem when font is not attached in mkv.

 was mentioned in:
 http://forum.videolan.org/viewtopic.php?f=14&t=51096&p=166340
 http://forum.videolan.org/viewtopic.php?f=7&t=50534&p=163900
"