krb5: fix CVE-2015-2695

lib/gssapi/spnego/spnego_mech.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 SPNEGO packet that is mishandled during
a gss_inquire_context call.

Backport upstream commit to fix it:
https://github.com/krb5/krb5/commit/b51b33f2bc5d1497ddf5bd107f791c101695000d



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