Add shutdown checks in dns_catz_dbupdate_callback()

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.

Closes #4171 (closed)