Prevent crash while reloading previously invalid but now valid DNS zone.
    
    The crash happened under these circumstances:
    - a DNS zone (test.) has NS record relative to this zone (ns.test.)
    - the name pointed to by NS record did not have any A/AAAA records
    - restart BIND to enforce zone load
    - zone loading failed because of invalid NS record
    - missing A/AAAA record for name ns.test. was added
    -> CRASH!
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/166