kex: Correctly check for the rekey (amends 83f2ac4a)

This is more reliable fix than 83f2ac4a, which was failing for the
server initialized rekeying, because the session_state is altered
by receiving the KEXINIT from server.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/kex.c

@@ -745,7 +745,7 @@ int ssh_set_client_kex(ssh_session session)
     }
 
     /* For rekeying, skip the extension negotiation */
-    if (session->session_state == SSH_SESSION_STATE_AUTHENTICATED) {
+    if (session->flags & SSH_SESSION_FLAG_AUTHENTICATED) {
         return SSH_OK;
     }