Fix post-registration SASL not working due to change from a few days ago.

(commit 0cf0c0faa2)

This was caused by register_user() being called twice, while it should
only have been called if !IsUser().

Reported by ProgVal in https://bugs.unrealircd.org/view.php?id=6606
My BuildBot screen was also all red :D.

src/modules/sasl.c

@@ -195,8 +195,8 @@ CMD_FUNC(cmd_sasl)
 				sasl_failed(target);
 			else if (*parv[4] == 'S')
 				sasl_succeeded(target);
-			/* Now that SASL response came in, check if handshake is finished */
-			if (is_handshake_finished(target))
+			/* Now that SASL response came in, check if user is unregistered and handshake finished now */
+			if (!IsUser(target) && is_handshake_finished(target))
 			{
 				register_user(target);
 				/* User MAY be killed now, that's okay, we don't deal with 'target' below */