Possible use after release

[Shlomo Pongratz]

In xio_conn.c, function xio_conn_close.

kref_put may call xio_conn_delayed_close which may free the conn, however conn is referenced later.

 kref_put(&conn->kref, xio_conn_delayed_close); <- can free.

 if (observer) {
  xio_conn_notify_observer(
    conn, observer,
    XIO_CONN_EVENT_CLOSED, NULL); <- possible use after free.

  xio_conn_unhash_observer(conn, observer);
  xio_conn_unreg_observer(conn, observer);
 }