Avoid possible race condition that kill OpenVPN itself

Avoid possible race condition that kill OpenVPN itself

If for whatever reason the child pid is zero, we would kill ourselves
since killing 0 means killing the own process group.

Reported-By: Joshua Rogers <contact@joshua.hu>
Found-By: Zeropath
Change-Id: I7b94de92723f9528b01cb932bb079eedf0f1f272
Signed-off-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1319
Message-Id: <20251027213308.5588-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg33910.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>