Fix task timing race in setnsec3param()
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 0127ba64)
Closes #2636 (closed)