avcodec/scpr: Factor some indexes out in decode_run_i()

avcodec/scpr: Factor some indexes out in decode_run_i()

This improves the speed of decode_run_i()

After:  clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SCPR_fuzzer-5656821117747200 in 13516 ms
Before: clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SCPR_fuzzer-5656821117747200 in 14018 ms

Improves: 11270/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SCPR_fuzzer-5656821117747200

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