libstdc++: Fix N3344 behavior on _Safe_iterator::_M_can_advance

We shall be able to advance from a 0 offset a value-initialized iterator.

libstdc++-v3/ChangeLog:

	* include/debug/safe_iterator.tcc (_Safe_iterator<>::_M_can_advance):
	Accept 0 offset advance on value-initialized iterator.
	* testsuite/23_containers/vector/debug/n3644.cc: New test case.