Segfault during echo-msg processing

[selvanair]

OpenVPN gui crashes if an echo-message directive is given with msg-notify and no msg text. For notify we use ShowTrayBalloon which cannot handle null message content.

To reproduce, add the following to the ovpn file:

echo msg "some text to display"
echo msg-notify MyTitle

This displays a notification as expected. Comment out the first line, and GUI segfaults.

[selvanair]

Fixed by #773