Fix KDC heap corruption vuln [CVE-2012-1015]

Fix KDC heap corruption vulnerability [MITKRB5-SA-2012-001
CVE-2012-1015].  The cleanup code in
kdc_handle_protected_negotiation() in kdc_util.c could free an
uninitialized pointer in some error conditions involving "similar"
enctypes and a failure in krb5_c_make_checksum().

Additionally, adjust the handling of "similar" enctypes to avoid
advertising enctypes that could lead to inadvertent triggering of this
vulnerability (possibly in unpatched KDCs).

Note that CVE-2012-1014 (also described in MITKRB5-SA-2012-001) only
applies to the krb5-1.10 branch and doesn't affect the master branch
or releases prior to krb5-1.10.

https://github.com/krb5/krb5/commit/3551501359c6d2396fa4779d378ae165f5b37242
Author: Tom Yu <tlyu@mit.edu>
Commit: 3551501359c6d2396fa4779d378ae165f5b37242
Branch: master
 src/kdc/kdc_preauth.c     |    3 ++-
 src/kdc/kdc_util.c        |    1 +
 src/lib/kdb/kdb_default.c |    3 +++
 3 files changed, 6 insertions(+), 1 deletions(-)