Implement NULL iterators for <forward_list> and <deque> re: N3644

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@187805 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/deque b/include/deque
index 6c0216d..8627272 100644
--- a/include/deque
+++ b/include/deque
@@ -278,7 +278,11 @@
     typedef random_access_iterator_tag  iterator_category;
     typedef _Reference                  reference;
 
-    _LIBCPP_INLINE_VISIBILITY __deque_iterator() _NOEXCEPT {}
+    _LIBCPP_INLINE_VISIBILITY __deque_iterator() _NOEXCEPT
+#if _LIBCPP_STD_VER > 11
+     : __m_iter_(nullptr), __ptr_(nullptr)
+#endif
+     {}
 
     template <class _Pp, class _Rp, class _MP>
     _LIBCPP_INLINE_VISIBILITY