libc sctp: improve conformance of sctp_getpaddrs()

When there is no association, don't return -1 and indicate ENOENT,
but return 0 instead. This is specified in RFC 6458.

PR:		260117
MFC after:	1 week

lib/libc/net/sctp_sys_calls.c

@@ -405,8 +405,12 @@ sctp_getpaddrs(int sd, sctp_assoc_t id, struct sockaddr **raddrs)
 	opt_len = (socklen_t)sizeof(uint32_t);
 	if (getsockopt(sd, IPPROTO_SCTP, SCTP_GET_REMOTE_ADDR_SIZE,
 	    &size_of_addresses, &opt_len) != 0) {
+		if (errno == ENOENT) {
+			return (0);
+		} else {
 			return (-1);
 		}
+	}
 	opt_len = (socklen_t)((size_t)size_of_addresses + sizeof(struct sctp_getaddresses));
 	addrs = calloc(1, (size_t)opt_len);
 	if (addrs == NULL) {