libhb: fix p-to-p start for some streams

The primary problem was in setting our "zero" time in reader based on a
stream that is not decoded.  Since this stream never reaches sync, there
would appear to be a long initial frame from syncs perspective.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6506 b64f7644-9d1e-0410-96f1-a4d463321fa5