avcodec/scpr: optimize shift loop.

Speeds code up from 50sec to 15sec

Fixes Timeout
Fixes: 3242/clusterfuzz-testcase-5811951672229888

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