sctp: remove unnecessary NULL check in sctp_ulpq_tail_event()

After commit 013b96ec ("sctp: Pass sk_buff_head explicitly to
sctp_ulpq_tail_event().") there is one more unneeded check of
skb_list for 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/20221019180735.161388-2-aleksei.kodanev@bell-sw.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sctp/ulpqueue.c

@@ -256,10 +256,7 @@ int sctp_ulpq_tail_event(struct sctp_ulpq *ulpq, struct sk_buff_head *skb_list)
 	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;
 }