device: Deal with links that vanish during initialization

nm_device_get_hw_address() may return NULL and nm_platform_link_get_type may
return NM_LINK_TYPE_NONE. While it might be a good idea to check for such cases
at the init time it seems easier to just ignore it and prevent blowing up in
subsequent deactivation.

A quick test case:

  # while :; do ip link add moo0 type veth peer moo1; ip link del moo0 ; done

Yields:

  NetworkManager:ERROR:devices/nm-device-ethernet.c:268:constructor:
    assertion failed: (link_type == NM_LINK_TYPE_ETHERNET ||
    link_type == NM_LINK_TYPE_VETH)

  nm_device_set_hw_addr: assertion 'addr != NULL' failed

https://bugzilla.gnome.org/show_bug.cgi?id=740992
Merge branch 'lr-veth-rm', remote-tracking branches 'fd/lr/ipv6-tokens', 'fd/lr/veth-managed', 'fd/lr/no-udev' and 'fd/lr/vpn-fixes' into integration