Update status for CVE-2018-19591/glibc on stretch

The issue was introduced by the specific commit

https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=2180fee114b778515b3f560e5ff1e795282e60b0

which potentially was backported to other release branches. This is not
the case at least for current stretch version. Some details given by
Florian Weimer in

https://www.openwall.com/lists/oss-security/2018/11/27/3