avcodec/pixlet: Fix runtime error: signed integer overflow: 2147483647 + 32 cannot be represented in type 'int'

Fixes: 1829/clusterfuzz-testcase-minimized-5527165321871360

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