OssFuzz: Do not read/use undefined values in UndoXYB
OssFuzz: Do not read/use undefined values in UndoXYB

Refactor code to extract common part. This allows to deal with MSAN in one place (instread of 6).
All ops are lane-wise, so it is fine to use unpoison-process-poison here.
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl