USN-6658-2: libxml2 vulnerability

USN-6658-1 fixed a vulnerability in libxml2. This update
provides the corresponding updates for Ubuntu 14.04 LTS,
Ubuntu 16.04 LTS, and Ubuntu 18.04 LTS.

Original advisory details:

It was discovered that libxml2 incorrectly handled certain XML documents. A
remote attacker could possibly use this issue to cause libxml2 to crash,
resulting in a denial of service, or possibly execute arbitrary code.

advisory attacker crash denial of service documents issue lts service ubuntu update updates usn vulnerability xml