USN-6403-3: libvpx vulnerabilities

USN-6403-1 fixed several vulnerabilities in libvpx. This update provides
the corresponding update for Ubuntu 16.04 LTS.

Original advisory details:

It was discovered that libvpx did not properly handle certain malformed
media files. If an application using libvpx opened a specially crafted
file, a remote attacker could cause a denial of service, or possibly
execute arbitrary code.

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