libxml2 regression

USN-2214-1 fixed vulnerabilities in libxml2. The upstream fix introduced a
regression when using xmllint with the --postvalid option. This update
fixes the problem.

We apologize for the inconvenience.

Original advisory details:

Daniel Berrange discovered that libxml2 would incorrectly perform entity
substitution even when requested not to. If a user or automated system were
tricked into opening a specially crafted document, an attacker could
possibly cause resource consumption, resulting in a denial of service.