When getting a TreeOwner that is null, make sure we don't crash trying to call QueryInterface on it when grabbing the nsIBaseWindow interface. r=saari

dom/src/base/nsGlobalWindow.cpp

@@ -3247,6 +3247,11 @@ NS_IMETHODIMP GlobalWindowImpl::GetTreeOwner(nsIBaseWindow** aTreeOwner)
 
    nsCOMPtr<nsIDocShellTreeOwner> treeOwner;
    docShellAsItem->GetTreeOwner(getter_AddRefs(treeOwner));
+   if(!treeOwner)
+      {
+      *aTreeOwner = nsnull;
+      return NS_OK;
+      }
 
    return CallQueryInterface(treeOwner, aTreeOwner);
 }