decutf8sub: fix a bug when passing subtitles through to MP4.

The decoder was setting the last character but forgot to increment out->size,
which led to weird artifacts being displayed when playing in e.g. VLC.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5346 b64f7644-9d1e-0410-96f1-a4d463321fa5