dnssec-checkds: cleanup memory on error paths

dnssec-checkds: cleanup memory on error paths

Move and give unique names to the dns_db_t, dns_dbnode_t and
dns_dbversion_t pointers, so they have global scope and therefore
are visible to cleanup.  Unique names are not strictly necessary,
as none of the functions involved call each other.

Change free_db to handle NULL pointers and also an optional
(dns_dbversion_t **).

In match_keyset_dsset and free_keytable, ki to be handled
differently to prevent a false positive NULL pointer dereference
warning from scan.

In formatset moved dns_master_styledestroy earlier and freed
buf before calling check_result to prevent memory leak.

In append_new_ds_set freed ds on the default path before
calling check_result to prevent memory leak.