avcodec/pixlet: Fixes: runtime error: signed integer overflow: 9203954323419769657 + 29897660706736950 cannot be represented in type 'long'

Fixes: 1569/clusterfuzz-testcase-minimized-6328690508038144

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