sctp: remove unnecessary NULL checks in sctp_enqueue_event()

After commit 178ca044 ("sctp: Make sctp_enqueue_event tak an
skb list."), skb_list cannot be NULL.

Detected using the static analysis tool - Svace.
Signed-off-by: Alexey Kodanev <aleksei.kodanev@bell-sw.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20221019180735.161388-3-aleksei.kodanev@bell-sw.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sctp/stream_interleave.c

@@ -490,11 +490,8 @@ static int sctp_enqueue_event(struct sctp_ulpq *ulpq,
 	if (!sctp_ulpevent_is_enabled(event, ulpq->asoc->subscribe))
 		goto out_free;
 
-	if (skb_list)
-		skb_queue_splice_tail_init(skb_list,
-					   &sk->sk_receive_queue);
-	else
-		__skb_queue_tail(&sk->sk_receive_queue, skb);
+	skb_queue_splice_tail_init(skb_list,
+				   &sk->sk_receive_queue);
 
 	if (!sp->data_ready_signalled) {
 		sp->data_ready_signalled = 1;
@@ -504,10 +501,7 @@ static int sctp_enqueue_event(struct sctp_ulpq *ulpq,
 	return 1;
 
 out_free:
-	if (skb_list)
-		sctp_queue_purge_ulpevents(skb_list);
-	else
-		sctp_ulpevent_free(event);
+	sctp_queue_purge_ulpevents(skb_list);
 
 	return 0;
 }