libx11 vulnerability

USN-6168-1 fixed a vulnerability in libx11. This update provides
the corresponding update for Ubuntu 14.04 ESM, Ubuntu 16.04 ESM,
and Ubuntu 18.04 ESM.

Original advisory details:

Gregory James Duck discovered that libx11 incorrectly handled certain
Request, Event, or Error IDs. If a user were tricked into connecting to a
malicious X Server, a remote attacker could possibly use this issue to
cause libx11 to crash, resulting in a denial of service.