avcodec/h264_cabac: runtime error: signed integer overflow: 2147483647 + 14 cannot be represented in type 'int'

Fixes: 614/clusterfuzz-testcase-4931860079575040

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