re PR libstdc++/57465 (Failed postcondition for std::function constructed with null function pointer)

	PR libstdc++/57465
	* include/std/functional
	(_Function_base::_Base_manager::_M_not_empty_function): Fix overload
	for pointers.
	* testsuite/20_util/function/cons/57465.cc: New.

From-SVN: r202974