USN-5606-2: poppler regression

Web: https://ubuntu.com/security/notices/USN-5606-2

USN-5606-1 fixed a vulnerability in poppler. Unfortunately it was missing a
commit to fix it properly. This update provides
the corresponding fix for Ubuntu 18.04 LTS and Ubuntu 16.04 ESM.

We apologize for the inconvenience.

Original advisory details:

It was discovered that poppler incorrectly handled certain
PDF. An attacker could possibly use this issue to cause a
denial of service or execute arbitrary code.