tls-crypt: don't leak memory for incorrect tls-crypt messages

tls-crypt: don't leak memory for incorrect tls-crypt messages

If tls_crypt_unwrap() failed, we would jump to cleanup and forget to free
the buffer.  Instead, allocate the buffer through gc, which is free'd in
the cleanup section.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170816170450.10415-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15282.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit fca89379c53fe2c145db96a5bcd32327c4bcfa78)