libwebp vulnerability

USN-6078-1 fixed a vulnerability in libwebp. This update
provides the corresponding update for Ubuntu 16.04 LTS.

Original advisory details:

Irvan Kurniawan discovered that libwebp incorrectly handled certain memory
operations. 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.