MetaWindow: don't set focus to unmanaged window

Closing the last window causes the no-focus window to gain
focus, which causes mutter to unfocus the just closed window,
and that crashes. As focusing a window that is about to be
destroyed does not make sense, avoid a crash in this case.

Note: this is probably a reference counting bug in MetaWayland
actually.

src/core/window.c

@@ -7399,6 +7399,9 @@ void
 meta_window_set_focused_internal (MetaWindow *window,
                                   gboolean    focused)
 {
+  if (window->unmanaging)
+    return;
+
   if (focused)
     {
       window->has_focus = TRUE;