Fix shutdown races in catzs

Fix shutdown races in catzs

The dns__catz_update_cb() does not expect that 'catzs->zones'
can become NULL during shutdown.

Add similar checks in the dns__catz_update_cb() and dns_catz_zone_get()
functions to protect from such a case. Also add an INSIST in the
dns_catz_zone_add() function to explicitly state that such a case
is not expected there, because that function is called only during a
reconfiguration.