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.

https://github.com/krb5/krb5/commit/5fca279ca4d18f1b5798847a98e7df8737d2eb7c
Author: Greg Hudson <ghudson@mit.edu>
Commit: 5fca279ca4d18f1b5798847a98e7df8737d2eb7c
Branch: master
 src/kadmin/server/misc.c |    4 +++-
 src/tests/t_policy.py    |    5 ++++-
 2 files changed, 7 insertions(+), 2 deletions(-)