avcodec/pixlet: Fix reading invalid numbers of bits

Fixes: asertion failure
Fixes: 1664/clusterfuzz-testcase-minimized-6587801187385344

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