h264: fix detection of optional trailing PPS elements

The PPS may contain a few trailing elements whose presence is
only signalled by data remaining after the the mandatory part
has been parsed.  The current code fails to take into account
the rbsp_trailing_bits() when deciding whether to parse these
optional elements.  Assuming no unnecessary padding bytes are
passed to this function, the optional elements are present if
either more than 8 extra bits remain or the remaining bits do
not form a valid rbsp_trailing_bits() after the mandatory PPS
elements have been parsed.

Signed-off-by: Mans Rullgard <mans@mansr.com>
