Core/Auth: Per SRP6a protocol, terminate connection of A % N

@@ -477,7 +477,7 @@ bool AuthSession::HandleLogonProof()
    A.SetBinary(logonProof->A, 32);

    // SRP safeguard: abort if A == 0
    if (A.IsZero())
    if ((A % N).IsZero())
        return false;

    SHA1Hash sha;