Phonet: fix oops in phonet_address_del() on non-Phonet device

A NULL dereference would occur when trying to delete an addres from a
network device that does not have any Phonet address.
Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/phonet/pn_dev.c

@@ -115,7 +115,7 @@ int phonet_address_del(struct net_device *dev, u8 addr)
 	pnd = __phonet_get(dev);
 	if (!pnd || !test_and_clear_bit(addr >> 2, pnd->addrs))
 		err = -EADDRNOTAVAIL;
-	if (bitmap_empty(pnd->addrs, 64))
+	else if (bitmap_empty(pnd->addrs, 64))
 		__phonet_device_free(pnd);
 	spin_unlock_bh(&pndevs.lock);
 	return err;