Committee can be smaller than CommitteeMembersCount

[roman-khimov]

Describe the bug
#2695 introduced a policy check for committee members and while it solves the main problem there is one potential problem with it.

neo/src/neo/SmartContract/Native/NeoToken.cs

Lines 404 to 411 in ed68274

	var candidates = GetCandidates(snapshot);
	if (voterTurnout < EffectiveVoterTurnout || candidates.Length < settings.CommitteeMembersCount)
	return settings.StandbyCommittee.Select(p => (p, candidates.FirstOrDefault(k => k.PublicKey.Equals(p)).Votes));
	return candidates
	.OrderByDescending(p => p.Votes)
	.ThenBy(p => p.PublicKey)
	.Where(p => !Policy.IsBlocked(snapshot, Contract.CreateSignatureRedeemScript(p.PublicKey).ToScriptHash()))
	.Take(settings.CommitteeMembersCount);

If we're to have X CommitteeMembersCount with X candidates we'll get past the check on line 405, but some candidates can now be kicked out of the list at line 410. IIUC in this case we'll end up with a committee that is smaller than intended (and potentially even lower number of CNs than intended. It's hard to predict how this will affect node's behavior.

This is not likely to happen in reality, but still should be taken into account.

Potential solution
Filter out bad keys right after GetCandidates. I doubt we can change GetCandidates itself.