USN-4969-2: DHCP vulnerability

USN-4969-1 fixed a vulnerability in DHCP. This update provides the corresponding update for Ubuntu 14.04 ESM and 16.04 ESM. Original advisory details: Jon Franklin and Pawel Wieczorkiewicz discovered that DHCP incorrectly handled lease file parsing. A remote attacker could possibly use this issue to cause DHCP to crash, resulting in a denial of service.