xorg-server vulnerabilities

USN-5193-1 fixed several vulnerabilities in X.Org. This update provides
the corresponding update for Ubuntu 14.04 ESM and Ubuntu 16.04 ESM.

Original advisory details:

Jan-Niklas Sohn discovered that the X.Org X Server incorrectly handled
certain inputs. An attacker could use this issue to cause the server to
crash, resulting in a denial of service, or possibly execute arbitrary
code and escalate privileges.