Replace kludgy assert with something a little less awful, and avoid needless...

  for (i=0; i < smartlist_len(desc->intro_nodes); ++i) {

    rend_intro_point_t *intro = smartlist_get(desc->intro_nodes, i);

    char ipoint[HEX_DIGEST_LEN+2];

    const size_t ipoint_len = HEX_DIGEST_LEN+1;

    ipoint[0] = '$';

    base16_encode(ipoint+1, HEX_DIGEST_LEN+1,

                  intro->extend_info->identity_digest,

                  DIGEST_LEN);

    tor_assert(buflen + *str_out >= cp); /* XXX021 This assert is a kludge. */

    strlcpy(cp, ipoint, buflen-(cp-*str_out));

    cp += strlen(ipoint)+1;

    tor_assert(strlen(ipoint) == ipoint_len);

    /* Assert that appending ipoint and its NUL won't over overun the

     * buffer. */

    tor_assert(cp + ipoint_len+1 < *str_out + buflen);

    memcpy(cp, ipoint, ipoint_len+1);

    cp += ipoint_len+1;

  }

  note_crypto_pk_op(REND_SERVER);

  r = crypto_pk_private_sign_digest(key, cp, *str_out, cp-*str_out);