avcodec/pixlet: Fixes: undefined shift in av_mod_uintp2()

Fixes: runtime error: shift exponent 4294967289 is too large for 32-bit type 'int'
Fixes: 3030/clusterfuzz-testcase-minimized-4649809254285312

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