sss_combine
ret = sss_combine(shares_, K - 1, &secret);
ret = sss_combine(shares_, K, &secret);
ret = sss_combine(shares_, K + 1, &secret);
ret = sss_combine(shares_, K + 1, &secret);
ret = sss_combine(shares_, K, &secret);
ret = sss_combine(shares_, K, &secret);
ATTRIBUTE_WARN_UNUSED_RET int sss_combine(const sss_share *shares, unsigned short k, sss_secret *secret);