UPS: check response data before using it

The uc_ups_fulfill_order_validate() function uses $response_obj->data without checking whether the data property is set. It would be better to check it and show an error message if there is no data (for example in case of network errors).

  $response_obj = drupal_http_request(variable_get('uc_ups_connection_address', ... );
  $response = new SimpleXMLElement($response_obj->data);