Add new busybox issue CVE-2018-20679

There is as well CVE-2019-5747 which got assigned for an initial
incomplete fix for CVE-2018-20679. Thus not affecting any released
version in Debian.

When fixing CVE-2018-20679 it is just important to apply the complete
fix to not open up CVE-2019-5747 itself.

Add respective notes to the CVE-2018-20679 entry.