Fix kadmin min_life check with nonexistent policy

Fix kadmin min_life check with nonexistent policy

In kadmind, self-service key changes require a check against the
policy's min_life field.  If the policy does not exist, this check
should succeed according to the semantics introduced by ticket #7385.
Fix check_min_life() to return 0 if kadm5_get_policy() returns
KADM5_UNK_POLICY.  Reported by John Devitofranceschi.

(cherry picked from commit 5fca279ca4d18f1b5798847a98e7df8737d2eb7c)

ticket: 8427
version_fixed: 1.14.3