Check for null kadm5 policy name [CVE-2015-8630]

In kadm5_create_principal_3() and kadm5_modify_principal(), check for
entry->policy being null when KADM5_POLICY is included in the mask.

CVE-2015-8630:

In MIT krb5 1.12 and later, an authenticated attacker with permission
to modify a principal entry can cause kadmind to dereference a null
pointer by supplying a null policy value but including KADM5_POLICY in
the mask.

    CVSSv2 Vector: AV:N/AC:H/Au:S/C:N/I:N/A:C/E:POC/RL:OF/RC:C

https://github.com/krb5/krb5/commit/b863de7fbf080b15e347a736fdda0a82d42f4f6b
Author: Greg Hudson <ghudson@mit.edu>
Commit: b863de7fbf080b15e347a736fdda0a82d42f4f6b
Branch: master
 src/lib/kadm5/srv/svr_principal.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)