redzone(9) documents that it skips MemGuard allocations, and the free and
realloc paths rely on a MemGuard pointer never being redzone-wrapped. With
DEBUG_MEMGUARD and DEBUG_REDZONE both set, the UMA zone-sampling path broke
this: redzone_setup() shifted the pointer into MemGuard's metadata, so the
later memguard_free() or memguard_realloc() panicked.

Skip redzone setup for a MemGuard address, matching the by-type path and
the documented behavior.