Fix minor leak in kdb5_util purge_mkeys

In kdb5_purge_mkeys(), if krb5_dbe_update_mkey_aux() fails, use the
cleanup label to free any allocated memory instead of returning
immediately.  Reported by Bean Zhang.

https://github.com/krb5/krb5/commit/6d822841648761ac888b9a95693ad87a8aaf165c
Author: Greg Hudson <ghudson@mit.edu>
Commit: 6d822841648761ac888b9a95693ad87a8aaf165c
Branch: master
 src/kadmin/dbutil/kdb5_mkey.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)