avcodec/bmp: Fix runtime error: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself

There is code checking height and width later, leaving an invalid value invalid
is thus fine.

Fixes: 635/clusterfuzz-testcase-6225161437052928
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
