libhb: fix another case of spurious "missing start code" messages
I have a poorly mastered BD that does not pad the adaptation field
of the PCR PID properly.  Since these packets are only used to extract
the PCR, we can drop them early before the code that would generate
these error messages.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5862 b64f7644-9d1e-0410-96f1-a4d463321fa5