lwtunnel: check return value of nla_nest_start

Function nla_nest_start() may return a NULL pointer on error. However,
in function lwtunnel_fill_encap(), the return value of nla_nest_start()
is not validated before it is used. This patch checks the return value
of nla_nest_start() against NULL.
Signed-off-by: NPan Bian <bianpan2016@163.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/lwtunnel.c

@@ -214,6 +214,8 @@ int lwtunnel_fill_encap(struct sk_buff *skb, struct lwtunnel_state *lwtstate)
 
 	ret = -EOPNOTSUPP;
 	nest = nla_nest_start(skb, RTA_ENCAP);
+	if (!nest)
+		goto nla_put_failure;
 	rcu_read_lock();
 	ops = rcu_dereference(lwtun_encaps[lwtstate->type]);
 	if (likely(ops && ops->fill_encap))