Mark CVE-2019-18684 as unimportant (as non-(security)-issue)

An attack is only viable if the attacker can write to fd/3. In the
concrete case fd/3 would point to /etc/sudoers. Then the only way to
write to /proc/$pid/fd/3 would be to have write permission to
/etc/sudoers itself.

Thanks: Todd C. Miller for the analysis.