xwm: Only call set_fullscreen if the window is mapped

If we get a fullscreen client message before we map the window, we don't
have a shell surface yet.  Just dont call set_fullscreen yet, and rely on
xserver_map_shell_surface() to fullscreen it when we map it.

src/xwayland/window-manager.c

@@ -1084,6 +1084,8 @@ weston_wm_window_handle_state(struct weston_wm_window *window,
 		if (window->fullscreen) {
 			window->saved_width = window->width;
 			window->saved_height = window->height;
+
+			if (window->shsurf)
 				shell_interface->set_fullscreen(window->shsurf,
 								WL_SHELL_SURFACE_FULLSCREEN_METHOD_DEFAULT,
 								0, NULL);