USN-6391-2: CUPS vulnerability

USN-6391-1 fixed a vulnerability in CUPS. This update provides
the corresponding update for Ubuntu 16.04 LTS and Ubuntu 18.04 LTS.

Original advisory details:

It was discovered that CUPS incorrectly parsed certain Postscript objects.
If a user or automated system were tricked into printing a specially
crafted document, a remote attacker could use this issue to cause CUPS to
crash, resulting in a denial of service, or possibly execute arbitrary
code.

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