[Bluetooth] Fix NULL pointer dereference in HCI line discipline

Normally a serial Bluetooth device is opened, TIOSETD'ed to N_HCI line
discipline, HCIUARTSETPROTO'ed and finally closed. In case the device
fails to HCIUARTSETPROTO, closing it produces a NULL pointer dereference.
Signed-off-by: NOhad Ben-Cohen <ohad@bencohen.org>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/hci_ldisc.c

@@ -307,7 +307,9 @@ static void hci_uart_tty_close(struct tty_struct *tty)
 
 	if (hu) {
 		struct hci_dev *hdev = hu->hdev;
-		hci_uart_close(hdev);
+
+		if (hdev)
+			hci_uart_close(hdev);
 
 		if (test_and_clear_bit(HCI_UART_PROTO_SET, &hu->flags)) {
 			hu->proto->close(hu);