libwebp vulnerabilities

USN-4971-1 fixed several vulnerabilities in libwebp. This update provides
the corresponding update for Ubuntu 14.04 ESM and Ubuntu 16.04 ESM.

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.