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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>
(cherry picked from commit fde5c7dc

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