window: Don't call drag_offer_handler if it's NULL

Reported by canbaby@21cn.com.

clients/window.c

@@ -1330,9 +1330,11 @@ display_handle_global(struct wl_display *display, uint32_t id,
 	} else if (strcmp(interface, "shm") == 0) {
 		d->shm = wl_shm_create(display, id);
 	} else if (strcmp(interface, "drag_offer") == 0) {
+		if (d->drag_offer_handler) {
 			offer = wl_drag_offer_create(display, id);
 			d->drag_offer_handler(offer, d);
 		}
+	}
 }
 
 static const char socket_name[] = "\0wayland";