Update information on CVE-2019-10714/imagemagick

The issue is not present up to the current version in unstable as the
issue was introduced while fixing upstrema issue 1455, which is only in
6.9.10-25 upstream. The fix for that introduced the problematic code in
magick/locale.c triggering the issue as reported for CVE-2019-10714.

Unless for unstable imagemagick will be updated to something between
6.9.10-25 and 6.9.10-32 the issue will not be present in Debian.