repair --dev null breakage caused by db950be85d37

repair --dev null breakage caused by db950be85d37

"make check" self-test was broken after commit db950be85d37 due to
do_ifconfig_setenv() not checking whether tt->did_ifconfig_setup
was set (which isn't, for "dev null" type setups)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <1432404098-29401-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9718
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 970c4bd2e473f625699bd56db44c1970a9e10ed9)