OSV-2026-711 Use-of-uninitialized-value in jxl::PerformMulBlending

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=511400706 Crash type: Use-of-uninitialized-value Crash state: jxl::PerformMulBlending jxl::PerformBlending jxl::NAVX2::BlendingStage::ProcessRow...