nl802154: fix Oops in ieee802154_nl_get_dev

ieee802154_nl_get_dev() lacks check for the existance of the device
that was returned by dev_get_XXX, thus resulting in Oops for non-existing
devices. Fix it.
Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>

net/ieee802154/netlink.c

@@ -276,6 +276,9 @@ static struct net_device *ieee802154_nl_get_dev(struct genl_info *info)
 	else
 		return NULL;
 
+	if (!dev)
+		return NULL;
+
 	if (dev->type != ARPHRD_IEEE802154) {
 		dev_put(dev);
 		return NULL;