sctp: sctp_sock_filter(): avoid list_entry() on possibly empty list

Use list_is_first() to check whether tsp->asoc matches the first
element of ep->asocs, as the list is not guaranteed to have an entry.

Fixes: 8f840e4 ("sctp: add the sctp_diag.c file")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20230208-sctp-filter-v2-1-6e1f4017f326@diag.uniroma1.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: pietroborrello

net/sctp/diag.c

@@ -343,11 +343,9 @@ static int sctp_sock_filter(struct sctp_endpoint *ep, struct sctp_transport *tsp
 	struct sctp_comm_param *commp = p;
 	struct sock *sk = ep->base.sk;
 	const struct inet_diag_req_v2 *r = commp->r;
-	struct sctp_association *assoc =
-		list_entry(ep->asocs.next, struct sctp_association, asocs);
 
 	/* find the ep only once through the transports by this condition */
-	if (tsp->asoc != assoc)
+	if (!list_is_first(&tsp->asoc->asocs, &ep->asocs))
 		return 0;
 
 	if (r->sdiag_family != AF_UNSPEC && sk->sk_family != r->sdiag_family)