USN-5034-2: c-ares vulnerability

USN-5034-1 fixed a vulnerability in c-ares. This update provides the corresponding update for Ubuntu 16.04 ESM. Original advisory details: Philipp Jeitner and Haya Shulman discovered that c-ares incorrectly validated certain hostnames returned by DNS servers. A remote attacker could possibly use this issue to perform Domain Hijacking attacks.