Avoid accessing freed memory when being replaced

If we are replaced via the window manager selection, then we close
the MetaDisplay before meta_finalize() runs. We should be careful
not to try to close the display again (and access already freed
memory) in that case, so clear the global 'the_display' variable.

See Metacity bug http://bugzilla.gnome.org/show_bug.cgi?id=588119

http://bugzilla.gnome.org/show_bug.cgi?id=592742

src/core/display.c

@@ -1007,6 +1007,7 @@ meta_display_close (MetaDisplay *display,
     meta_compositor_destroy (display->compositor);
   
   g_object_unref (display);
+  the_display = NULL;
 
   meta_quit (META_EXIT_SUCCESS);
 }

src/core/main.c

@@ -427,8 +427,11 @@ meta_select_display (gchar *display_name)
 static void
 meta_finalize (void)
 {
-  meta_display_close (meta_get_display (),
-                      CurrentTime); /* I doubt correct timestamps matter here */
+  MetaDisplay *display = meta_get_display ();
+
+  if (display)
+    meta_display_close (display,
+                        CurrentTime); /* I doubt correct timestamps matter here */
 
   meta_session_shutdown ();
 }