Bluetooth: Fix possible NULL dereference in hci_uart_tty_receive

This patch adds a NULL check for the HCI UART ldisc driver because some
of HCI UART drivers allow hci_uart_tty_receive function to be called
even though the HCI device hasn't been registered yet.
Signed-off-by: Chan-yeol Park <chanyeol.park@samsung.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

drivers/bluetooth/hci_ldisc.c

@@ -388,7 +388,10 @@ static void hci_uart_tty_receive(struct tty_struct *tty, const u8 *data, char *f
 
 	spin_lock(&hu->rx_lock);
 	hu->proto->recv(hu, (void *) data, count);
+
+	if (hu->hdev)
 		hu->hdev->stat.byte_rx += count;
+
 	spin_unlock(&hu->rx_lock);
 
 	tty_unthrottle(tty);