[PATCH] USB: make sure we never reference a usbserial port after it has been unregistered.

drivers/usb/serial/usb-serial.c

@@ -871,8 +871,10 @@ static void destroy_serial (struct kobject *kobj)
 
 	/* the ports are cleaned up and released in port_release() */
 	for (i = 0; i < serial->num_ports; ++i)
-		if (serial->port[i]->dev.parent != NULL)
+		if (serial->port[i]->dev.parent != NULL) {
 			device_unregister(&serial->port[i]->dev);
+			serial->port[i] = NULL;
+		}
 
 	/* If this is a "fake" port, we have to clean it up here, as it will
 	 * not get cleaned up in port_release() as it was never registered with