h264: Fix maximum reference count check for non-b frames

      Below fixes the maximum reference count check for second reference list in
non-B frames.  There is nothing to prohibit full (field sized) reference
list in this case as far as I can tell, and this fixes several syntax-test
files here (this is a regression caused when this check was made more
stringent by
http://git.videolan.org/?p=ffmpeg.git;a=commit;h=dc9ce40069bde3d28f8d0b3e5bd733ae255fecb5)

Probably a silly corner case seldom seen irl, but thought I'd pass along
in case there was interest in correcting the check.

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h264: Fix maximum reference count check for non-b frames; full range is
technically ok

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>