Fix GSS krb5 GSS_C_DELEG_FLAG ret_flags result

The krb5 gss_accept_sec_context could erroneously return
GSS_C_DELEG_FLAG in ret_flags if either:

* The token included the GSS_C_DELEG_FLAG but did not include at least
  28 bytes in the authenticator checksum.
* The initial token included the GSS_C_DELEG_FLAG but a DCE-style
  exchange was performed.

Fix both of these cases.

(cherry picked from commit d899084e24555dc8fd091eb08187ca2a45a08324)

https://github.com/krb5/krb5/commit/7b4bf661ecc39459f78665a356858de9f6daaabc
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 7b4bf661ecc39459f78665a356858de9f6daaabc
Branch: krb5-1.13
 src/lib/gssapi/krb5/accept_sec_context.c |   19 ++++++++++---------
 1 files changed, 10 insertions(+), 9 deletions(-)