af_mpls: add null dev check in find_outdev

This patch adds null dev check for the 'cfg->rc_via_table ==
NEIGH_LINK_TABLE or dev_get_by_index() failed' case
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mpls/af_mpls.c

@@ -411,6 +411,9 @@ static struct net_device *find_outdev(struct net *net,
 		dev = dev_get_by_index(net, cfg->rc_ifindex);
 	}
 
+	if (!dev)
+		return ERR_PTR(-ENODEV);
+
 	return dev;
 }