[fix] Only add defined callbacks to the stack (#447)

Before that commit, undefined callbacks were also added to the array,
which could lead to 'Maximum call stack size exceeded' error when
flush() method was called.

Fixes #399

lib/socket.js

@@ -393,8 +393,8 @@ Socket.prototype.sendPacket = function (type, data, options, callback) {
 
     this.writeBuffer.push(packet);
 
-    // add send callback to object
-    this.packetsFn.push(callback);
+    // add send callback to object, if defined
+    if (callback) this.packetsFn.push(callback);
 
     this.flush();
   }