krb5: fix CVE-2015-2696

lib/gssapi/krb5/iakerb.c in MIT Kerberos 5 (aka krb5) before 1.14
relies on an inappropriate context handle, which allows remote
attackers to cause a denial of service (incorrect pointer read and
process crash) via a crafted IAKERB packet that is mishandled during
a gss_inquire_context call.

Backport upstream commit to fix it:
https://github.com/krb5/krb5/commit/e04f0283516e80d2f93366e0d479d13c9b5c8c2a



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