Core/Auth: Per SRP6a protocol, terminate connection of A % N == 0. This resolves another authentication bypass issue

(cherry picked from commit cmangos/mangos-wotlk@ea3e08a812)

src/server/authserver/Server/AuthSession.cpp

@@ -479,7 +479,7 @@ 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;