shell: Only assign focus on click if there is a pointer focus surface

If we don't have a background image from the desktop-shell client or the
pointer for some other reason doesn't have a focus we trigger a
segfault as we try to deref the seat->pointer->focus NULL pointer.

https://bugs.freedesktop.org/show_bug.cgi?id=73066

clients/desktop-shell.c

@@ -765,7 +765,7 @@ background_configure(void *data,
 	struct background *background =
 		(struct background *) window_get_user_data(window);
 
-	widget_schedule_resize(background->widget, width, height);
+	widget_schedule_resize(background->widget, width - 256, height);
 }
 
 static void