dlog proofs bug

[omershlo]

Currently we follow multi-party-ecdsa logic of two dlog proofs
see here: https://github.com/ZenGo-X/fs-dkr/pull/17/files#diff-386a609f746d74ecbc3e04d7ae0e82e2e7a58a72a16908d2fbbb955b9d355971R73

There is a bug in this logic, as can be explained and fixed here:
ZenGo-X/multi-party-ecdsa#145 (comment)

please apply the same fix here as well

[DmytroTym]

Yup, I'll make a PR. Sorry once again.