When setnsec3param() is schedule from zone_postload() there's no
guarantee that `zone->db` is not `NULL` yet. Thus when the
setnsec3param() is called, we need to check for `zone->db` existence and
reschedule the task, because calling `rss_post()` on a zone with empty
`.db` ends up with no-op (the function just returns).
(cherry picked from commit 0127ba6472)
cb6bfd1e9c