USN-4996-2: OpenEXR vulnerabilities

USN-4996-1 fixed several vulnerabilities in OpenEXR. This update provides the corresponding update for Ubuntu 16.04 ESM. Original advisory details: It was discovered that OpenEXR incorrectly handled certain malformed EXR image files. If a user were tricked into opening a crafted EXR image file, a remote attacker could cause a denial of service, or possibly execute arbitrary code.