We have a Heimdal special where when the acceptor sends back an error
token for clock skew or ticket-not-yet-valid errors then the acceptor
application will get GSS_S_CONTINUE_NEEDED from gss_accept_sec_context()
so that the initiator may retry with the same context.
But we were retaining the auth_context, which means that when the
initiator does send a new token, the acceptor leaks memory because
krb5_verify_ap_req2() doesn't clean up the auth_context on reuse. The
end result is that we leak a lot in those cases.
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