Fix crash on startup when user manually deleted servers from gui-config.json

shadowsocks-csharp/Model/Configuration.cs

@@ -54,6 +54,9 @@ namespace Shadowsocks.Model
                 string configContent = File.ReadAllText(CONFIG_FILE);
                 Configuration config = JsonConvert.DeserializeObject<Configuration>(configContent);
                 config.isDefault = false;
+
+                if (config.configs.Count == 0)
+                    config.configs.Add(GetDefaultServer());
                 if (config.localPort == 0)
                     config.localPort = 1080;
                 if (config.index == -1 && config.strategy == null)