libhb: work around libav decoder error durint p-to-p encoding

In some cases, initial data when in p-to-p mode causes libav decoder
initialization to fail.  This only happens when multi-threaded encoding
is enabled.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6229 b64f7644-9d1e-0410-96f1-a4d463321fa5