USN-5718-2: pixman vulnerability

USN-5718-1 fixed a vulnerability in pixman. This update provides the
corresponding update for Ubuntu 14.04 ESM and Ubuntu 16.04 ESM.

Original advisory details:

Maddie Stone discovered that pixman incorrectly handled certain memory
operations. A remote attacker could use this issue to cause pixman to
crash, resulting in a denial of service, or possibly execute arbitrary
code.

usn vulnerability