Fix the adding of padding to COOKIE-ECHO chunks.

Thanks to Mark Wodrich who found this issue while fuzz testing the
usrsctp stack and reported the issue in
sctplab/usrsctp#382

MFC after:		3 days

sys/netinet/sctp_output.c

@@ -9059,8 +9059,7 @@ sctp_send_cookie_echo(struct mbuf *m,
 				pad = 4 - pad;
 			}
 			if (pad > 0) {
-				cookie = sctp_pad_lastmbuf(cookie, pad, NULL);
-				if (cookie == NULL) {
+				if (sctp_pad_lastmbuf(cookie, pad, NULL) == NULL) {
 					return (-8);
 				}
 			}