decssasub, dectx3gsub: fix writing past hb_buffer_t data[size - 1].

This is not safe and causes subtitle corruption.

Same fix as SVN revision 5346 for the remaining text-based subtitle deocders.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5390 b64f7644-9d1e-0410-96f1-a4d463321fa5