USN-6369-2: libwebp vulnerability

USN-6369-1 fixed a vulnerability in libwebp. This update provides the
corresponding update for Ubuntu 18.04 LTS.

Original advisory details:

It was discovered that libwebp incorrectly handled certain malformed
images. If a user or automated system were tricked into opening a
specially crafted image file, a remote attacker could use this issue to
cause libwebp to crash, resulting in a denial of service, or possibly
execute arbitrary code.

advisory attacker automated crash file image images issue libwebp lts malformed system ubuntu update usn vulnerability