app: don't dereference NULL image pointer

windows_menu_display_query_tooltip(): bail out if "image" is
NULL. Can't happen currently but did happen temporarily while hacking
on related code. Better safe than sorry.

app/menus/windows-menu.c

@@ -415,6 +415,9 @@ windows_menu_display_query_tooltip (GtkWidget  *widget,
   gint       width;
   gint       height;
 
+  if (! image)
+    return;
+
   text = gtk_widget_get_tooltip_text (widget);
   gtk_tooltip_set_text (tooltip, text);
   g_free (text);