The dns__catz_update_cb() function was earlier updated (see
d2ecff3c4a) to use a separate
'dns_db_t' object ('catz->updb' instead of 'catz->db') to
avoid a race between the 'dns__catz_update_cb()' and
'dns_catz_dbupdate_callback()' functions, but the 'REQUIRE'
check there still checks the validity of the 'catz->db' object.
Fix the omission.
a2817541b3