avcodec/scpr: mask bits to prevent out of array read

Fixes: 1615/clusterfuzz-testcase-minimized-6625214647500800

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