avcodec/magicyuv: Check that vlc len is not too large

Fixes: runtime error: shift exponent -95 is negative
Fixes: 2568/clusterfuzz-testcase-minimized-4926115716005888

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