CVE-2016-2019: libcli/smb: don't allow guest sessions if we require signing

Note real anonymous sessions (with "" as username) don't hit this
as we don't even call smb2cli_session_set_session_key() in that case.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11860

Signed-off-by: Stefan Metzmacher <metze@samba.org>
