USN-4969-3: DHCP regression

USN-4969-1 fixed a vulnerability in DHCP. The package for Ubuntu 21.04 introduced a regression causing it to reject certain valid configuration files. This update fixes the problem. We apologize for the inconvenience. 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.