Only set std::enable_shared_from_this member once.

Only set std::enable_shared_from_this member once.

	* include/bits/shared_ptr.h (__enable_shared_from_this_helper): Use
	nullptr.
	* include/bits/shared_ptr_base.h (weak_ptr::_M_assign): Don't assign
	if ownership is already shared with a shared_ptr object.
	(__enable_shared_from_this_helper): Use nullptr.
	* testsuite/20_util/enable_shared_from_this/members/const.cc: New.
	* testsuite/20_util/enable_shared_from_this/members/reinit.cc: New.
	* testsuite/20_util/enable_shared_from_this/requirements/
	explicit_instantiation.cc: Instantiate with const and incomplete types.

From-SVN: r227232