h264parse: fix collection of access units to preserve config headers.

h264parse: fix collection of access units to preserve config headers.

Always use a GstAdapter when collecting access units (alignment="au")
in either byte-stream or avcC format. This is required to properly
preserve config headers like SPS and PPS when invalid or broken NAL
units are subsequently parsed.

More precisely, this fixes scenario like:
<SPS> <PPS> <invalid-NAL> <slice>

where we used to reset the output frame buffer when an invalid or
broken NAL is parsed, i.e. SPS and PPS NAL units were lost, thus
preventing the next slice unit to be decoded, should this also
represent any valid data.

https://bugzilla.gnome.org/show_bug.cgi?id=732203

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>