krb5: fix CVE-2014-5351

The kadm5_randkey_principal_3 function in lib/kadm5/srv/svr_principal.c
in kadmind in MIT Kerberos 5 (aka krb5) before 1.13 sends old keys in a
response to a -randkey -keepold request, which allows remote authentic-
ated users to forge tickets by leveraging administrative access.

This back-ported patch fixes CVE-2014-5351.

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>