Issue #2532 - Fix a null pointer crash while doing dialog form submission

See Bug 1652699

dom/html/HTMLFormSubmission.cpp

@@ -957,7 +957,7 @@ HTMLFormSubmission::GetFromForm(nsGenericHTMLElement* aForm,
     // If there isn't one, or if it does not have an open attribute, do
     // nothing.
     if (!dialog || !dialog->Open()) {
-      return NS_OK;
+      return NS_ERROR_FAILURE;
     }
 
     nsAutoString result;