dco_linux: avoid bogus text when netlink message is not parsed

dco_linux: avoid bogus text when netlink message is not parsed

We may bail out parsing an incoming DCO message because it may
concern a different interface.
In that case we print the following debug messages:

    dco_do_read
    ovpn-dco: ignoring message (type=5) for foreign ifindex 313
    process_incoming_dco: received message of type 0 - ignoring

However, the last message is confusing, because there is no message
of type 0 being received, but the message_type was simply not
initialized.

Bail out parsing earlier and avoid printing any bogus text.

Change-Id: I568faa12a5960e8b69de23c2df413b70b231592c
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250523073848.20848-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31763.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>