alpha-modifier-v1: add a missing NULL check in wlr_alpha_modifier_v1_get_surface_state()

types/wlr_alpha_modifier_v1.c

@@ -181,5 +181,8 @@ struct wlr_alpha_modifier_v1 *wlr_alpha_modifier_v1_create(struct wl_display *di
 const struct wlr_alpha_modifier_surface_v1_state *wlr_alpha_modifier_v1_get_surface_state(
 		struct wlr_surface *wlr_surface) {
 	struct wlr_alpha_modifier_surface_v1 *surface = surface_from_wlr_surface(wlr_surface);
+	if (surface == NULL) {
+		return NULL;
+	}
 	return &surface->current;
 }