[Bluetooth] Avoid NULL pointer dereference with tty->driver

This patch checks for tty->driver before trying to call flush_buffer().

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
index 1994270..93ba25b 100644
--- a/drivers/bluetooth/hci_ldisc.c
+++ b/drivers/bluetooth/hci_ldisc.c
@@ -191,7 +191,7 @@
 
 	/* Flush any pending characters in the driver and discipline. */
 	tty_ldisc_flush(tty);
-	if (tty->driver->flush_buffer)
+	if (tty->driver && tty->driver->flush_buffer)
 		tty->driver->flush_buffer(tty);
 
 	if (test_bit(HCI_UART_PROTO_SET, &hu->flags))
@@ -290,7 +290,7 @@
 	if (tty->ldisc.flush_buffer)
 		tty->ldisc.flush_buffer(tty);
 
-	if (tty->driver->flush_buffer)
+	if (tty->driver && tty->driver->flush_buffer)
 		tty->driver->flush_buffer(tty);
 
 	return 0;