dco-win: Fix crash when cancelling pending operation

dco-win: Fix crash when cancelling pending operation

The OVERLAPPED structure must remain valid for the entire duration of an
asynchronous operation. Previously, when a TCP connection was pending
inside the NEW_PEER call, the OVERLAPPED structure was defined as a
local variable within dco_p2p_new_peer().

When CancelIo() was called later from close_tun_handle(), the OVERLAPPED
structure was already out of scope, resulting in undefined behavior and
stack corruption.

This fix moves the OVERLAPPED structure to the tuntap struct, ensuring
it remains valid throughout the operation's lifetime.

Github: closes OpenVPN/openvpn#715

Change-Id: Ib1db457c42a80f6b8fc0e3ceb4a895d4cf7f0155
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250401181535.7854-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59168247/
URL: https://gerrit.openvpn.net/c/openvpn/+/928
Signed-off-by: Gert Doering <gert@greenie.muc.de>