avcodec/scpr: Fix multiple runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'

Fixes: 1422/clusterfuzz-testcase-minimized-5030993939398656

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