auth:credentials: avoid talloc_reference in cli_credentials_set_netlogon_creds()

Typically cli_credentials_set_netlogon_creds() should be used directly
before the DCERPC bind. And cli_credentials_get_netlogon_creds()
should be only used by the gensec layer, which only needs a copy.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Oct 16 23:56:01 CEST 2013 on sn-devel-104