Fix UBSAN's cry afoul

Fix UBSAN's cry afoul

Technically, we weren't actually doing this the way C wants us to,
legally.  The zmemcpy's turn into NOPs for pretty much all > 0
optimization levels and this gets us defined behavior with the
sanitizer, putting the optimized load by arbitrary alignment into the
compiler's hands instead of ours.

Backport note: Replaced zmemcpy with direct memcpy, as that is what we
end up with in a later commit anyway.