use self ref in end() and destroy()

Signed-off-by: Mario Kozjak <kozjakm1@gmail.com>

lib/telnet-client.js

@@ -118,15 +118,19 @@ Telnet.prototype.exec = function(cmd, opts, callback) {
 }
 
 Telnet.prototype.end = function() {
+  var self = this
+
   return new Promise(function(resolve) {
-    this.telnetSocket.end()
+    self.telnetSocket.end()
     resolve()
   })
 }
 
 Telnet.prototype.destroy = function() {
+  var self = this
+
   return new Promise(function(resolve) {
-    this.telnetSocket.destroy()
+    self.telnetSocket.destroy()
     resolve()
   })
 }