Fix crash while showing config dialog.

If user manually deleted servers from gui-config.json

shadowsocks-csharp/View/ConfigForm.cs

@@ -151,7 +151,7 @@ namespace Shadowsocks.View
             _modifiedConfiguration = controller.GetConfigurationCopy();
             LoadConfiguration(_modifiedConfiguration);
             _lastSelectedIndex = _modifiedConfiguration.index;
-            if (_lastSelectedIndex < 0)
+            if (_lastSelectedIndex < 0 || _lastSelectedIndex >= ServersListBox.Items.Count)
             {
                 _lastSelectedIndex = 0;
             }