Subtitles: fix missing CEA-708 closed captions.

This is a quick hack version of
7d9bf3193e0f3d1795c1ead6f16aa386ab7de587, which fixes a CEA-708
decoding bug exposed by recent changes to NBC's CEA-708 encoding.
(cherry picked from commit eca41c857ef9618a2e8335b90e97583e08858107)