Instead of a raw pointer use a combination of Option and NonNull which
is exactly the same size as a raw pointer, expect you cannot access a
nullpointer by accident.
The assembly is completly identical on the hot path (the one that does not
procude a panic). Only one move more on the cold path (because of the
unwrap message (?).

Changed `*mut ...` to `Option<NonNull<...>>`

See merge request !1