Rather than assert'ing in such case, emit warning if the length of a NAL unit
is less than expected 2 and discard it.
Based on patch by Benjamin M. Schwartz <bens@alum.mit.edu>
Fixes#650416.
Functionally equivalent to (legacy)h264parse and re-uses the latter's low
level NAL parsing, but otherwise based on GstBaseParse, and replacing
some property configuration with caps negotiation.