Invalid pointer dereference in `fmt::Pointer` impl for `Atomic` and `Shared` when the underlying pointer is invalid

Affected versions of fmt::Display dereference the underlying pointer. This causes a invalid pointer dereference e.g., when a pointer created with Atomic::null or Shared::null. fmt::Debug impls and pre-0.9 fmt::Display impls, which do not dereference pointers, are not affected by this issue...