dkg: fix 2 participants

[conradoplg]

dkg gets stuck if trying to use 2 participants. Fix it.

(I guess it would even try to work with a single participant, we should return an error in that case)

The log was:

frost-client dkg -d "ZF demo group 2" -s frostd.zfnd.org -S e76d2f673eb91aca53d6054c0f5271c25790cf6c15805ff3324ee2b83ab96c6d,97e89b49517f97972c827c76ca4872eb6341ee5d3b99b0cae9d4b748c12bc67d -t 2 -C redpallas
Logging in...
Creating DKG session...
Getting session info...
Waiting for other participants to send their Round 1 Packages...............
Waiting for other participants to send their broadcasted Round 1 Packages...................................^C

It's very likely that this is related to the broadcast channel, since in some parts it needs to wait for n-2 messages.