dco_linux: validate tun interface before fetching stats

dco_linux: validate tun interface before fetching stats

If dco_get_peer_stats() is called with an uninitialized c->c1.tuntap it
results in a segfault. This issue happens when a client who has not
connected to any server:
  - has --management and exits,
  - has --management and a management interface client issues either
    `bytecount` or `status` or
  - if SIGUSR2 is sent to it.

Add a check to ensure the tun interface was set up before attempting to
retrieve peer statistics.

Change-Id: I40c11864745cc1619cb9cbf490b168f90feb5eac
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250902164521.23145-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32768.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>