User wants to use that server after selected it in the ConfigForm. If user deletes
the server in use, we can't find its index, then the previous logic will revert to
index 0, which is not expected behaviour.
Since we handled 'ServersListBox.SelectedIndex' in various event handlers, we can
trust it all the time.
Signed-off-by: Syrone Wong <wong.syrone@gmail.com>