conf.c: fix unprivileged network case

If unprivileged users are using a veth nic, then ifindex is still 0
at lxc_assign_network() (because lxc_create_network() was skipped).
So check for that case before we use lxc->ifindex to decide if we
have an empty network namespace.

We probably should change the !netdev->ifindex check to a
netdev->type == LXC_NET_EMPTY check, but I've been making enough
mistakes today not to risk that.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>