zone_resigninc(): check whether zone->db is a valid pointer before attaching
The zone_resigninc() function does not check the validity of 'zone->db', which can crash named if the zone was unloaded earlier, for example with "rndc delete".
Check that 'zone->db' is not 'NULL' before attaching to it, like it is done in zone_sign() and zone_nsec3chain() functions, which can similarly be called by zone maintenance.
Closes #4054 (closed)