USN-4957-2: DjVuLibre vulnerabilities

USN-4957-1 fixed several vulnerabilities in DjVuLibre. This update provides the corresponding update for Ubuntu 16.04 ESM. Original advisory details: It was discovered that DjVuLibre incorrectly handled certain memory operations. If a user or automated system were tricked into processing a specially crafted DjVu file, a remote attacker could cause applications to hang or crash, resulting in a denial of service, or possibly execute arbitrary code.