[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: Ohad Ben-Cohen <ohad@bencohen.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
index 75c1508..e8ae0d7 100644
--- a/drivers/bluetooth/hci_ldisc.c
+++ b/drivers/bluetooth/hci_ldisc.c
@@ -307,7 +307,9 @@
 
 	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);