avcodec/h264_ps: Check delta scale for validity

Fixes: signed integer overflow: 5 + 2147483646 cannot be represented in type 'int'
Fixes: 634/clusterfuzz-testcase-5285420445204480

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