lxc_spawn: initialize netpipe

When rebooting an unprivileged container, netpipe starts out
as not -1.  If count_veths somehow changed this could lead
to trying to send data over nonexistent pipe.  (Ok can't
*really* happen, as it currently stands, but it's an open
end)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>