avcodec/ylc: Fix shift overflow

Fixes: runtime error: shift exponent 32 is too large for 32-bit type 'unsigned int'
Fixes: 2698/clusterfuzz-testcase-minimized-4713541443518464

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