gssapi: Fix the return flags if we don't have delegation credentials.

The section 5.1 in RFC2744 for the ret_flags states:

GSS_C_DELEG_FLAG
  True - Delegated credentials are available via the
  delegated_cred_handle parameter.

So it should not be set if the credentials are not available.

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I've found this testing the Samba GSSAPI implementation using MIT Kerberos. In Samba I've added an additional NULL check for the deleg_cred value now.

https://git.samba.org/?p=asn/samba.git;a=commitdiff;h=f5f39355ca98d462053efdf8811dfc5ccfaa1532