Make sure response->extra_data.data is always cleared out

Make sure response->extra_data.data is always cleared out

Otherwise a bad read can sometimes cause the function to return -1 with
an invalid pointer in extra_data.data, which is attempted to be freed
by the caller (e.g. libwbclient/wbc_pam.c wbcAuthenticateUserEx())
by calling winbindd_free_response().

Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>