Resolve "Assertion failure in dns__catz_update_cb() on shutdown"

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.

Closes #4381 (closed)

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