ssl_openssl: fix compiler warning by removing getbio() wrapper

ssl_openssl: fix compiler warning by removing getbio() wrapper

An API change in openssl 1.1 made the BIO_METHOD * returned by BIO_f_ssl()
and BIO_s_mem() const, as well as the BIO_METHOD * argment of BIO_new()
const.  This meant that our getbio() function would either have an API
inconsistent with 1.0 or 1.1.

The wrapper was basically an ASSERT, so fix this by replacing the wrapper
with an ASSERT.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1513246897-28171-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16083.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 006d6a57b8835c15222359bfb42c95005723394c)