package/samba4: security bump to version 4.9.4

package/samba4: security bump to version 4.9.4

Fixes the following security issues:

- CVE-2018-14629 dns: Fix CNAME loop prevention using counter regression
- CVE-2018-16853: Fix S4U2Self crash with MIT KDC build
- CVE-2018-16853: Do not segfault if client is not set

For more info, see the release notes:
https://www.samba.org/samba/history/samba-4.9.4.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Peter: mention security impact, add CVE info]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>