libstdc++: Make deque iterator operator- usable with value-init iterators

libstdc++: Make deque iterator operator- usable with value-init iterators

N3644 implies that operator- can be used on value-init iterators. We now return
0 if both iterators are value initialized. If only one is value initialized we
keep the UB by returning the result of a normal computation which is a meaningless
value.

libstdc++-v3/ChangeLog:

	PR libstdc++/70303
	* include/bits/stl_deque.h (std::deque<>::operator-(iterator, iterator)):
	Return 0 if both iterators are value-initialized.
	* testsuite/23_containers/deque/70303.cc: New test.
	* testsuite/23_containers/vector/70303.cc: New test.