OSV-2026-872 Use-of-uninitialized-value in vpx_variance16x16_avx2

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=520181861 Crash type: Use-of-uninitialized-value Crash state: vpxvariance16x16avx2 vp8pickintramode vp8cxencodeintramacroblock...