xwm: fix a crash in handle_state

To produce the bug, build and run: (you don't need the game data to test)

https://github.com/clintbellanger/flare-engine/

$ mv ~/.config/flare ~/.config/flare.bak
$ ./flare  # click 'configure', set full screen mode then click 'ok'
           # weston will crash and dump core.

[Pekka: edited the commit message]
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

xwayland/window-manager.c

@@ -1326,7 +1326,9 @@ weston_wm_window_handle_state(struct weston_wm_window *window,
 								WL_SHELL_SURFACE_FULLSCREEN_METHOD_DEFAULT,
 								0, NULL);
 		} else {
+			if (window->shsurf)
 				shell_interface->set_toplevel(window->shsurf);
+
 			window->width = window->saved_width;
 			window->height = window->saved_height;
 			if (window->frame)