avcodec/h264_cabac: Fix runtime error: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself

Fixes: 858/clusterfuzz-testcase-5168477042114560

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