Protection for Dangling NS records

a cat of meti 
  sisoaitc.go.jp.		86400 IN NS ns1.xserver.jp.
  sisoaitc.go.jp.		86400 IN NS ns2.xserver.jp.
  sisoaitc.go.jp.		86400 IN NS ns3.xserver.jp.

from Dec 28 by tss @ suzuki.sist.chukyo-u.ac.jp