key_data is unused. Presumably this was copypasta from DB_seq in
hdb/db.c, where it is used for the Heimdal database format in which
the value may be missing a principal but it can be recovered from the
key. In contrast, the mit-krb5 format appears to always store the
principal in the value and _hdb_mdb_value2entry always retrieves it,
with no need for a key2principal fallback.
fix https://github.com/heimdal/heimdal/issues/1102
e3e271ff70