N3644 support for vector<bool>

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@187911 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__bit_reference b/include/__bit_reference
index 58d475e..857dd5a 100644
--- a/include/__bit_reference
+++ b/include/__bit_reference
@@ -1108,7 +1108,11 @@
     unsigned          __ctz_;
 
 public:
-    _LIBCPP_INLINE_VISIBILITY __bit_iterator() _NOEXCEPT {}
+    _LIBCPP_INLINE_VISIBILITY __bit_iterator() _NOEXCEPT
+#if _LIBCPP_STD_VER > 11
+    : __seg_(nullptr), __ctz_(0)
+#endif
+    {}
 
     _LIBCPP_INLINE_VISIBILITY
     __bit_iterator(const __bit_iterator<_Cp, false>& __it) _NOEXCEPT