[Bluetooth] More checks if DLC is still attached to the TTY

If the DLC device is no longer attached to the TTY device, then return
errors or default values for various callbacks of the TTY layer.
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/bluetooth/rfcomm/tty.c

@@ -697,9 +697,13 @@ static int rfcomm_tty_write_room(struct tty_struct *tty)
 
 	BT_DBG("tty %p", tty);
 
+	if (!dev || !dev->dlc)
+		return 0;
+
 	room = rfcomm_room(dev->dlc) - atomic_read(&dev->wmem_alloc);
 	if (room < 0)
 		room = 0;
+
 	return room;
 }
 
@@ -915,12 +919,14 @@ static void rfcomm_tty_unthrottle(struct tty_struct *tty)
 static int rfcomm_tty_chars_in_buffer(struct tty_struct *tty)
 {
 	struct rfcomm_dev *dev = (struct rfcomm_dev *) tty->driver_data;
-	struct rfcomm_dlc *dlc = dev->dlc;
 
 	BT_DBG("tty %p dev %p", tty, dev);
 
-	if (!skb_queue_empty(&dlc->tx_queue))
-		return dlc->mtu;
+	if (!dev || !dev->dlc)
+		return 0;
+
+	if (!skb_queue_empty(&dev->dlc->tx_queue))
+		return dev->dlc->mtu;
 
 	return 0;
 }
@@ -928,11 +934,12 @@ static int rfcomm_tty_chars_in_buffer(struct tty_struct *tty)
 static void rfcomm_tty_flush_buffer(struct tty_struct *tty)
 {
 	struct rfcomm_dev *dev = (struct rfcomm_dev *) tty->driver_data;
-	if (!dev)
-		return;
 
 	BT_DBG("tty %p dev %p", tty, dev);
 
+	if (!dev || !dev->dlc)
+		return;
+
 	skb_queue_purge(&dev->dlc->tx_queue);
 
 	if (test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags) && tty->ldisc.write_wakeup)
@@ -952,11 +959,12 @@ static void rfcomm_tty_wait_until_sent(struct tty_struct *tty, int timeout)
 static void rfcomm_tty_hangup(struct tty_struct *tty)
 {
 	struct rfcomm_dev *dev = (struct rfcomm_dev *) tty->driver_data;
-	if (!dev)
-		return;
 
 	BT_DBG("tty %p dev %p", tty, dev);
 
+	if (!dev)
+		return;
+
 	rfcomm_tty_flush_buffer(tty);
 
 	if (test_bit(RFCOMM_RELEASE_ONHUP, &dev->flags))