When parsing '--setenv opt xx ..' make sure a third parameter is present

When parsing '--setenv opt xx ..' make sure a third parameter is present

When no parameters are present, set it to "setenv opt" to trigger a
descriptive error message. And, thus get rid of the pesky NULL pointer
dereferencing.

Trac: #779

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1480470794-6349-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13311.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 997795353916ffcb413a2da02dc7f210fd621954)