libc sctp: fix sctp_getladdrs() when reporting no addresses

Section 9.5 of RFC 6458 (SCTP Socket API) requires that
sctp_getladdrs() returns 0 in case the socket is unbound. This
is the cause of reporting 0 addresses. So don't indicate an
error, just report this case as required.

PR: 260117
MFC after: 1 week