Connection: add guard for conditional

lib/Connection.js

@@ -1562,7 +1562,7 @@ Connection.prototype._enqueue = function(fullcmd, promote, cb) {
     // defer until next tick for requests like APPEND and FETCH where access to
     // the request object is needed immediately after enqueueing
     process.nextTick(function() { self._processQueue(); });
-  } else if (this._curReq.type === 'IDLE') {
+  } else if (this._curReq && this._curReq.type === 'IDLE') {
     this._idle.enabled = false;
     this.debug && this.debug('=> DONE');
     this._sock.write('DONE' + CRLF);