Don't potentially dereference NULL in net/dcb/dcbnl.c:dcbnl_getapp()

nla_nest_start() may return NULL. If it does then we'll blow up in
nla_nest_end() when we dereference the pointer.
Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dcb/dcbnl.c

@@ -626,6 +626,9 @@ static int dcbnl_getapp(struct net_device *netdev, struct nlattr **tb,
 	dcb->cmd = DCB_CMD_GAPP;
 
 	app_nest = nla_nest_start(dcbnl_skb, DCB_ATTR_APP);
+	if (!app_nest)
+		goto out_cancel;
+
 	ret = nla_put_u8(dcbnl_skb, DCB_APP_ATTR_IDTYPE, idtype);
 	if (ret)
 		goto out_cancel;