Fix "queryCallback is not a function"

Author: juliusza

lib/client.js

@@ -380,11 +380,15 @@ Client.prototype.query = function (config, values, callback) {
       var error = new Error('Query read timeout')
 
       this.emit('error', error)
-      queryCallback(error)
 
-      // we already returned an error,
-      // just do nothing if query completes
-      query.callback = () => {}
+      if (typeof queryCallback === 'function') {
+        queryCallback(error)
+
+        // we already returned an error,
+        // just do nothing if query completes
+        query.callback = () => {
+        }
+      }
 
       // Remove from queue
       var index = this.queryQueue.indexOf(query)