Bluetooth: hci_bcm: add missing tty-device sanity check

Make sure to check the tty-device pointer before looking up the sibling
platform device to avoid dereferencing a NULL-pointer when the tty is
one end of a Unix98 pty.

Fixes: 0395ffc1ee05 ("Bluetooth: hci_bcm: Add PM for BCM devices")
Cc: stable <stable@vger.kernel.org>     # 4.3
Cc: Frederic Danis <frederic.danis@linux.intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/hci_bcm.c

@@ -287,6 +287,9 @@ static int bcm_open(struct hci_uart *hu)
 
 	hu->priv = bcm;
 
+	if (!hu->tty->dev)
+		goto out;
+
 	mutex_lock(&bcm_device_lock);
 	list_for_each(p, &bcm_device_list) {
 		struct bcm_device *dev = list_entry(p, struct bcm_device, list);
@@ -307,7 +310,7 @@ static int bcm_open(struct hci_uart *hu)
 	}
 
 	mutex_unlock(&bcm_device_lock);
-
+out:
 	return 0;
 }