mshtml: Properly handle NULL VT_DISPATCH refChild in IHTMLDOMNode::insertBefore.

dlls/mshtml/htmlnode.c

@@ -513,6 +513,8 @@ static HRESULT WINAPI HTMLDOMNode_insertBefore(IHTMLDOMNode *iface, IHTMLDOMNode
     case VT_NULL:
         break;
     case VT_DISPATCH: {
+        if(!V_DISPATCH(&refChild))
+            break;
         ref_node = get_node_obj(This->doc, (IUnknown*)V_DISPATCH(&refChild));
         if(!ref_node) {
             ERR("unvalid node\n");

dlls/mshtml/tests/dom.c

@@ -5920,6 +5920,13 @@ static void test_create_elems(IHTMLDocument2 *doc)
     test_elem_innertext(body, "insert test");
     test_elem_innerhtml((IUnknown*)body, "insert test");
 
+    node = test_create_text(doc, " Test");
+    V_VT(&var) = VT_DISPATCH;
+    V_DISPATCH(&var) = NULL;
+    test_node_insertbefore((IUnknown*)body, node, &var);
+    test_elem_innertext(body, "insert test Test");
+    IHTMLDOMNode_Release(node);
+
     hres = IHTMLDocument2_QueryInterface(doc, &IID_IHTMLDocument5, (void**)&doc5);
     if(hres == S_OK)
     {