libavcodec/h264_parse: don't use uninitialized value when chroma_format_idc==0

When parsing a monochrome file, chroma_log2_weight_denom was used without
being initialized, which could lead to a bogus error message being printed, e.g.
  [h264 @ 0x61a000026480] chroma_log2_weight_denom 24576 is out of range
It also could led to warnings using AddressSanitizer.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
