Fix OpenSSL 1.1.1 not using auto elliptic curve selection

Fix OpenSSL 1.1.1 not using auto elliptic curve selection

Commit 8a01147ff attempted to avoid calling the deprecated/noop
operation SSL_CTX_set_ecdh_auto by surrounding it with #ifdef.
Unfortunately, that change also made the return; that would exit
the function no longer being compiled when using OpenSSL 1.1.0+.
As consequence OpenVPN with OpenSSL 1.1.0+ would always set
secp384r1 as ecdh curve unless otherwise specified by ecdh

This patch restores the correct/previous behaviour.
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20200328040858.16505-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19630.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit d8ac887c6b1b57a1953ab62058b4aed5d8c11f65)