[Bug c++/60168] Incorrect check in ~unique_ptr() when Deleter::pointer type is not a pointer type

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60168

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The standard also specifies the behaviour of ~unique_ptr<T,D> in terms of
comparing the stored pointer with nullptr.