[PATCH v1] nss: fix __get_default_domain logic

[Florian Weimer]

* Frederic Berat:

> I looked back at the commit (64d1e08ea822bf47cb2796ad0f727136227f983c)
> that introduced this function and I also confirm that this makes more
> sense this way.  The `__nss_get_default_domain` function replaced uses
> of `yp_get_default_domain` which has similar implementation and the
> appropriate `domainname[0] == '\0'` check.

DJ, please mention that commit in the commit message.

Thanks,
Florian