sctp: remove unnecessary NULL check in sctp_ulpq_tail_event()

After commit 013b96ec6461 ("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 <[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/ulpqueue.c

···
 	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;
 }