When CoreDNS starts up and can't get a zone transfer going the Apex is
empty. This `nil` is then transformed into wireformat, which fails with
a nil pointer dereference in Go DNS.

In this case we should just return SERVFAIL, because we don't have any
info (yet). Note the lookup code returned NXDOMAIN, which is correct
from a lookup standpoint, but also invalidates every name in the future
loaded zone.

Anyway, look for an apex before doing the lookup and return SERVFAIL if
nothing is found.

Fixes #679