Fix memory leak in buffer unit tests

Fix memory leak in buffer unit tests

buffer_list_push_data does not take ownership of the memory, so just feed
it stack data to plug the leak.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <1512724338-22197-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16055.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 2c7c760dfbddbc9cf348bce06fa922c1217a2039)