Don't launch a useless circuit in rend_client_reextend_intro_circuit

  o Major bugfixes:

    - Don't launch a useless circuit after failing to use one of a

      hidden service's introduction points.  Previously, we would

      launch a new introduction circuit, but not set the hidden

      service which that circuit was intended to connect to, so it

      would never actually be used.  A different piece of code would

      then create a new introduction circuit correctly, so this bug

      was harmless until it caused an assertion in the client-side

      part of the #3825 fix to fail.  Bug reported by katmagic and

      found by Sebastian Hahn.  Bugfix on 0.2.1.13-alpha; fixes bug

      4212.

    result = circuit_extend_to_new_exit(circ, extend_info);

  } else {

    log_info(LD_REND,

             "Building a new introduction circuit, this time to %s.",

             safe_str_client(extend_info_describe(extend_info)));

             "Closing intro circ %d (out of RELAY_EARLY cells).",

             circ->_base.n_circ_id);

    circuit_mark_for_close(TO_CIRCUIT(circ), END_CIRC_REASON_FINISHED);

    if (!circuit_launch_by_extend_info(CIRCUIT_PURPOSE_C_INTRODUCING,

                                       extend_info,

                                       CIRCLAUNCH_IS_INTERNAL)) {

      log_warn(LD_REND, "Building introduction circuit failed.");

      result = -1;

    } else {

    /* connection_ap_handshake_attach_circuit will launch a new intro circ. */

    result = 0;

  }

  }

  extend_info_free(extend_info);

  return result;

}