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.