OSV-2021-786 Heap-buffer-overflow in jxl::PerformAlphaWeightedAdd

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34518 Crash type: Heap-buffer-overflow WRITE Crash state: jxl::PerformAlphaWeightedAdd jxl::PerformBlending jxl::ImageBlender::RectBlender::DoBlending...