avcodec/scpr: Check y in first line loop in decompress_i()

      Fixes: out of array access
Fixes: 1478/clusterfuzz-testcase-minimized-5285486908145664

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