Use `connection._protocol._ended` state

lib/dialects/mysql/connection-manager.js

@@ -74,7 +74,7 @@ ConnectionManager.prototype.disconnect = function(connection) {
   });
 };
 ConnectionManager.prototype.validate = function(connection) {
-  return connection && connection.state !== 'disconnected';
+  return connection && ! connection._protocol._ended;
 };
 
 module.exports = ConnectionManager;