Fix a crash if a xul file specifies a non-existent start page (NS_ADDREF changed to NS_IF_ADDREF).

dom/src/base/nsGlobalWindow.cpp

@@ -290,7 +290,7 @@ NS_IMETHODIMP
 GlobalWindowImpl::GetDocument(nsIDOMDocument** aDocument)
 {
   *aDocument = mDocument;
-  NS_ADDREF(mDocument);
+  NS_IF_ADDREF(mDocument);
 
   return NS_OK;
 }