avcodec/pixlet: Fix runtime error: signed integer overflow: 436207616 * -5160230545260541 cannot be represented in type 'long'

Fixes: 1462/clusterfuzz-testcase-minimized-6558894463647744

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