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)