avcodec/hevc_cabac: Fix integer overflow in ff_hevc_cu_qp_delta_abs()

Fixes: signed integer overflow: 2147483647 + 1073741824 cannot be represented in type 'int'
Fixes: 4555/clusterfuzz-testcase-minimized-4505532481142784

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