container: _nmsg_container_serialize(): fix warning on old gcc's
container: _nmsg_container_serialize(): fix warning on old gcc's

nmsg/container.c: In function '_nmsg_container_serialize':
nmsg/container.c:122: warning: 'len_wire' may be used uninitialized in this function

on "gcc (GCC) 4.2.1 20070831 patched [FreeBSD]"

hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg