USN-4968-2: LZ4 vulnerability

USN-4968-1 fixed a vulnerability in LZ4. This update provides
the corresponding update for Ubuntu 14.04 ESM and Ubuntu 16.04 ESM.

Original advisory details:

It was discovered that LZ4 incorrectly handled certain memory operations.
If a user or automated system were tricked into uncompressing a specially-
crafted LZ4 file, a remote attacker could use this issue to cause LZ4 to
crash, resulting in a denial of service, or possibly execute arbitrary
code.