tipc: check nl sock before parsing nested attributes

Make sure the socket for which the user is listing publication exists
before parsing the socket netlink attributes.

Prior to this patch a call without any socket caused a NULL pointer
dereference in tipc_nl_publ_dump().

Tested-and-reported-by: Baozeng Ding <sploving1@gmail.com>
Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.cm>
Signed-off-by: David S. Miller <davem@davemloft.net>
static void tipc_sk_respond(struct sock *sk, struct sk_buff *skb, int err)
{
	u32 selector;
	u32 dnode;
	u32 onode = tipc_own_addr(sock_net(sk));

	if (!tipc_msg_reverse(onode, &skb, err))
		return;

	dnode = msg_destnode(buf_msg(skb));
	selector = msg_origport(buf_msg(skb));
	tipc_node_xmit_skb(sock_net(sk), skb, dnode, selector);
}

static void tipc_sk_respond(struct sock *sk, struct sk_buff *skb, int err)
{
	u32 selector;
	u32 dnode;
	u32 onode = tipc_own_addr(sock_net(sk));

	if (!tipc_msg_reverse(onode, &skb, err))
		return;

	dnode = msg_destnode(buf_msg(skb));
	selector = msg_origport(buf_msg(skb));
	tipc_node_xmit_skb(sock_net(sk), skb, dnode, selector);
}
