avcodec/scpr: Fix multiple runtime error: index 256 out of bounds for type 'unsigned int [256]'

Fixes: 1519/clusterfuzz-testcase-minimized-5286680976162816

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