Remove null check after checking for checking for did_open_tun

Remove null check after checking for checking for did_open_tun

If we indicate that the tun device has been opened the c1.tuntap struct
is guaranteed to be defined. This extra null check is something that
Coverity flags as we access a do a null check after already accessing fields
of tuntap

Change-Id: I9966636163c7dfa208d26f1cadbf5b81937f3a34
Signed-off-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240925151104.13036-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29447.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>