USN-5855-4: ImageMagick vulnerabilities

USN-5855-1 fixed vulnerabilities in ImageMagick. This update provides the
corresponding updates for Ubuntu 14.04 ESM and Ubuntu 16.04 ESM.

Original advisory details:

It was discovered that ImageMagick incorrectly handled certain PNG images.
If a user or automated system were tricked into opening a specially crafted
PNG file, an attacker could use this issue to cause ImageMagick to stop
responding, resulting in a denial of service, or possibly obtain the
contents of arbitrary files by including them into images.

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