[libcxx] [test] Fix an improper assumption about Null Forward Iterators.

Value-initialized iterators still can't be compared to those with parents.

Fixes D26626.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@289462 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/containers/sequences/list/iterators.pass.cpp

@@ -138,7 +138,6 @@ int main()
 #endif
 #if TEST_STD_VER > 11
     {
-        std::list<int> c;
         std::list<int>::iterator ii1{}, ii2{};
         std::list<int>::iterator ii4 = ii1;
         std::list<int>::const_iterator cii{};
@@ -151,9 +150,6 @@ int main()
         assert ( (cii == ii1 ));
         assert (!(ii1 != cii ));
         assert (!(cii != ii1 ));
-
-        assert ( ii1 != c.cbegin());
-        assert ( cii != c.begin());
     }
 #endif