Don't free cred handle used in kadm5 server handle

At the end of setup_gss(), gss_client_creds is released, but
this credential handle is saved in kadm5_server_handle_t in
handle->clnt->cl_auth->(struct rpc_gss_data *)ah_private->sec->cred
Accessing these credentials can result in use after free.

This fix stores credential reference in server handle and releases
the credentials in kadm5_destroy.