avcodec/pixlet: Fix shift exponent 4294967268 is too large for 32-bit type 'int'

Fixes: 1336/clusterfuzz-testcase-minimized-4761381930795008

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