CVE-2016-2111: s3:libsmb: don't send a raw NTLMv2 response when we want to use spnego

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11749

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
