CVE-2016-2110: auth/credentials: clear the LMv2 key for NTLMv2 in cli_credentials_get_ntlm_response()

If we clear CLI_CRED_LANMAN_AUTH and we should also clear the lm_response buffer
and don't send it over the net.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11644

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
