test for NULL

Author: huixie90

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.const/pointer_deleter.pass.cpp

@@ -121,6 +121,10 @@ int main(int, char**)
       auto deleter = [](auto pointer) { delete pointer; };
       std::shared_ptr<int> p(new int, deleter);
     }
+
+    {
+      std::shared_ptr<int> p(NULL, [](auto){});
+    }
 #endif
 
 #if TEST_STD_VER >= 17