OSV-2021-561 UNKNOWN WRITE in wuffs_base__pixel_swizzler__bgr__bgra_premul__src

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32451 Crash type: UNKNOWN WRITE Crash state: wuffsbasepixelswizzlerbgrbgrapremulsrc wuffsbasepixelswizzlerswizzleinterleavedfromslice fuzz...