kex: Do not negotiate extensions during rekey

The RFC 8308 clearly says, that the additional  ext-info-c  should
be added only to the first SSH_MSG_KEXINIT.

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

src/kex.c

@@ -742,6 +742,11 @@ int ssh_set_client_kex(ssh_session session)
         }
     }
 
+    /* For rekeying, skip the extension negotiation */
+    if (session->session_state == SSH_SESSION_STATE_AUTHENTICATED) {
+        return SSH_OK;
+    }
+
     /* Here we append  ext-info-c  to the list of kex algorithms */
     kex = client->methods[SSH_KEX];
     len = strlen(kex);