avcodec/magicyuv: Check len to be supported

Fixes: shift exponent -1 is negative
Fixes: 1390/clusterfuzz-testcase-minimized-5452757630713856

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