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connection_exit_connect_dir and connection_ap_make_link creates linked pair for connections that can be freed before connection_unlink thus invalidates linked_conn.

  connection_link_connections(partner, base_conn);

  if (connection_add(base_conn) < 0) { /* no space, forget it */
    connection_free(base_conn);
    return NULL;
  }

  connection_link_connections(TO_CONN(dirconn), TO_CONN(exitconn));

  if (connection_add(TO_CONN(exitconn))<0) {
    connection_edge_end(exitconn, END_STREAM_REASON_RESOURCELIMIT);
    connection_free(TO_CONN(exitconn));
    connection_free(TO_CONN(dirconn));
    return 0;
  }

  /* link exitconn to circ, now that we know we can use it. */
  exitconn->next_stream = circ->n_streams;
  circ->n_streams = exitconn;

  if (connection_add(TO_CONN(dirconn))<0) {
    connection_edge_end(exitconn, END_STREAM_REASON_RESOURCELIMIT);
    connection_close_immediate(TO_CONN(exitconn));
    connection_mark_for_close(TO_CONN(exitconn));
    connection_free(TO_CONN(dirconn));
    return 0;
  }

If connection_add fails then linked_conn from another connection is broken. (currently it can to fail only if BUFFEREVENTS used by code)