krb5: fix CVE-2015-2698

The iakerb_gss_export_sec_context function in lib/gssapi/krb5/iakerb.c
in MIT Kerberos 5 (aka krb5) 1.14 pre-release 2015-09-14 improperly
accesses a certain pointer, which allows remote authenticated users
to cause a denial of service (memory corruption) or possibly have
unspecified other impact by interacting with an application that calls
the gss_export_sec_context function. NOTE: this vulnerability exists
because of an incorrect fix for CVE-2015-2696.

Backport upstream commit to fix it:
https://github.com/krb5/krb5/commit/3db8dfec1ef50ddd78d6ba9503185995876a39fd



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