compat/lz4: Update to v1.9.2

compat/lz4: Update to v1.9.2

It's a long while since the bundled lz4 library has received an update.
It pulls in a lot of various fixes and enhancements, some of the changes
fixes compiler warnings and hardens the code a bit too.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20201001154658.9798-1-davids@openvpn.net>
URL: https://www.mail-archive.com/search?l=mid&q=20201001154658.9798-1-davids@openvpn.net
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 0f44a9080530df70410106c244e9efc7f2d8a802)