vector<bool> does addressof(*ptr) where ptr is returned by
allocate(n), but if n==0 that pointer might not be dereferencable.

While testing the fix I also found some bugs in the
__gnu_test::PointerBase helper that needed correcting.

	PR libstdc++/80893
	* include/bits/stl_bvector.h (vector<bool>::_M_initialize): Avoid
	null pointer dereference when size is zero.
	* testsuite/23_containers/vector/bool/80893.cc: New.
	* testsuite/util/testsuite_allocator.h (PointerBase::PointerBase):
	Add non-explicit constructor from nullptr.
	(PointerBase::derived() const): Add const-qualified overload.

Tested powerpc64le-linux, committed to trunk.