sctp: remove unnecessary NULL checks in sctp_enqueue_event()

After commit 178ca044aa60 ("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 <[e-mail]>
Reviewed-by: Xin Long <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sctp/stream_interleave.c

···
 	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;
···
 	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;
 }