window: Handle updating from no to no monitor

When we received two hot plug events that both resulted in headless
configuration, we tried to find a new window monitor given the old.
That resulted in a null pointer dereference; avoid that by only trying
to find the same monitor if there was an old one.

https://bugzilla.gnome.org/show_bug.cgi?id=788607

src/core/window.c

@@ -3793,7 +3793,7 @@ meta_window_update_for_monitors_changed (MetaWindow *window)
   new = find_monitor_by_winsys_id (window, window->preferred_output_winsys_id);
 
   /* Otherwise, try to find the old output on a new monitor */
-  if (!new)
+  if (old && !new)
     new = find_monitor_by_winsys_id (window, old->winsys_id);
 
   /* Fall back to primary if everything else failed */