desktop-shell: Don't crash on when attemping to switch

Attempting to perform a switch on a surface (already) closed will trip
the assert in activate(), so check if we have a weston_desktop_surface
before trying to activate it.

Fixes #543

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

desktop-shell/shell.c

@@ -4451,7 +4451,7 @@ switcher_destroy(struct switcher *switcher)
 		weston_surface_damage(view->surface);
 	}
 
-	if (switcher->current) {
+	if (switcher->current && get_shell_surface(switcher->current->surface)) {
 		activate(switcher->shell, switcher->current,
 			 keyboard->seat,
 			 WESTON_ACTIVATE_FLAG_CONFIGURE);