Technically, we're not allowed to take the address of a member can't
exist relative to the null pointer. That makes me wonder how any sane
compliant system implements the offsetof macro, but let's let sleeping
balrogs lie.
Fixes 13096; patch on 0.1.1.9-alpha; patch from "teor", who was using
clang -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error -ftrapv
