nsec3.c: Add a missing dns_db_detachnode() call
There is one case in 'dns_nsec3_activex()' where it returns but forgets to detach the db node. Add the missing 'dns_db_detachnode()' call.
This case only triggers if 'sig-signing-type' (privatetype) is set to 0 (which by default is not), or if the function is called with 'complete' is set to 'true' (which at this moment do not exist).
Closes #3500 (closed)