qpdf vulnerabilities

USN-5026-1 fixed several vulnerabilities in QPDF. This update provides
the corresponding update for Ubuntu 16.04 ESM.
Original advisory details:

It was discovered that QPDF incorrectly handled certain malformed PDF
files. A remote attacker could use this issue to cause QPDF to consume
resources, resulting in a denial of service. (CVE-2018-18020)

It was discovered that QPDF incorrectly handled certain malformed PDF
files. A remote attacker could use this issue to cause QPDF to crash,
resulting in a denial of service, or possibly execute arbitrary code.
(CVE-2021-36978)