Fix memory leak in x509_verify_cert_ku()

Fix memory leak in x509_verify_cert_ku()

If keyUsage was only required to be present, but no specific value was
required, we would omit to free the extracted string.  This happens as of
2.4.1, if --remote-cert-tls is used.  In that case we leak a bit of
memory on each TLS (re)negotiation.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1494154878-18403-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14563.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 7b94d3bbbea46efcea12e1df24da52fe508d0173)