Check whether zone->db is a valid pointer before attaching

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.

(cherry picked from commit fae0930eb84063fc03d711a0c772c58e5b470377)