avcodec/hevc_sei: Fix integer overflows in decode_nal_sei_message()

Fixes: signed integer overflow: 2147483520 + 255 cannot be represented in type 'int'
Fixes: 4554/clusterfuzz-testcase-minimized-4843714515042304

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