While nm_platform_link_get_ifindex() is documented to return 0 if the device
is not found, don't rely on it. Instead, check that a valid(!) ifindex was
returned, and only then set the ifindex. Otherwise leave it at zero. There
is of course no difference in practice, but we generally treat invalid ifindexes
as <= 0, so it's not immediately clear what nm_platform_link_get_ifindex()
returns to signal no device.