don't emit error on command if callback with error was invoked

Author: sidorares

lib/pool.js

@@ -130,9 +130,10 @@ Pool.prototype.query = function (sql, values, cb) {
 
   this.getConnection(function (err, conn) {
     if (err) {
-      cmdQuery.emit('error', err);
       if (typeof cmdQuery.onResult === 'function') {
         cmdQuery.onResult(err);
+      } else {
+        cmdQuery.emit('error', err);
       }
       return;
     }