window: Treat being headless as not monitor sized

When we are headless, treat this as if the window is always not monitor
sized. This might cause windows to temporarly become redirected while
being headless, but this is harmless as when a new monitor is
connected, we'll recalculate weather it should be redirected or not.

https://bugzilla.gnome.org/show_bug.cgi?id=787637

src/core/window.c

@@ -2904,6 +2904,9 @@ meta_window_is_screen_sized (MetaWindow *window)
 gboolean
 meta_window_is_monitor_sized (MetaWindow *window)
 {
+  if (!window->monitor)
+    return FALSE;
+
   if (window->fullscreen)
     return TRUE;