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)

https://github.com/krb5/krb5/commit/ed725b8e0f43d8e6cf0ebe4eea89edabef1eba3d
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: ed725b8e0f43d8e6cf0ebe4eea89edabef1eba3d
Branch: krb5-1.14
 src/kadmin/server/misc.c |    4 +++-
 src/tests/t_policy.py    |    5 ++++-
 2 files changed, 7 insertions(+), 2 deletions(-)