libhb: don't set duration of PGS subtitles

PGS subtitles are terminated by an explicit "end of sub" subtitle in
the stream.  The duration is not used (or known in advance). So every
PGS was getting the default 10s duration assigned.  This made the 
overall duration of the stream appear longer than it really is.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6384 b64f7644-9d1e-0410-96f1-a4d463321fa5