[Bluetooth] Check if RFCOMM session is still attached to the TTY

If the RFCOMM session is no longer attached to the TTY device, then it
makes no sense to go through with changing the termios settings.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/rfcomm/tty.c

@@ -765,7 +765,7 @@ static void rfcomm_tty_set_termios(struct tty_struct *tty, struct termios *old)
 
 	BT_DBG("tty %p termios %p", tty, old);
 
-	if (!dev)
+	if (!dev || !dev->dlc || !dev->dlc->session)
 		return;
 
 	/* Handle turning off CRTSCTS */