Fix gssrpc data leakage [CVE-2014-9423]

[MITKRB5-SA-2015-001] In svcauth_gss_accept_sec_context(), do not copy
bytes from the union context into the handle field we send to the
client.  We do not use this handle field, so just supply a fixed
string of "xxxx".

In gss_union_ctx_id_struct, remove the unused "interposer" field which
was causing part of the union context to remain uninitialized.

https://github.com/krb5/krb5/commit/5bb8a6b9c9eb8dd22bc9526751610aaa255ead9c
Author: Greg Hudson <ghudson@mit.edu>
Commit: 5bb8a6b9c9eb8dd22bc9526751610aaa255ead9c
Branch: master
 src/lib/gssapi/mechglue/mglueP.h |    1 -
 src/lib/rpc/svc_auth_gss.c       |   25 ++-----------------------
 2 files changed, 2 insertions(+), 24 deletions(-)