offset_of is unsound

[RalfJung]

The macro at

nix/src/macros.rs

Line 210 in 6af11c1

macro_rules! offset_of {

is unsound and causes UB due to using * on a NULL pointer. See the reference for details.

The memoffset crate provides a version of this macro that avoids UB (on older versions of rustc, this is not always possible, so it falls back to the 'least incorrect' version when needed).