libhb: fix a couple of problems with subtitle merging
I did not handle cases where it was impossible to determine the
stop time properly.

I also did not handle the last subitle properly.  It would have been lost.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5635 b64f7644-9d1e-0410-96f1-a4d463321fa5