When a zone database update callback is called, the 'catzs' object,
extracted from the callback argument, might be already shutting down,
in which case the 'catzs->zones' can be NULL and cause an assertion
failure when calling isc_ht_find().
Add an early return from the callback if 'catzs->shuttingdown' is true.
Also check the validity of 'catzs->zones' after locking 'catzs' in
case there is a race with dns_catz_shutdown_catzs() running in another
thread.
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