[chronojump] Better closing ping on exit

[Xavier de Blas <xaviblas src gnome org>]

commit ef719738cf7ce53ee92826925fc7677672a8b1d7
Author: Xavier de Blas <xaviblas gmail com>
Date:   Mon Jun 26 22:49:13 2017 +0200

    Better closing ping on exit

 src/json.cs |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/json.cs b/src/json.cs
index f762d11..c3307d1 100644
--- a/src/json.cs
+++ b/src/json.cs
@@ -202,7 +202,8 @@ public class Json
        public void PingAbort()
        {
                requestPingAborting = true;
-               requestPing.Abort(); //cancel an asynchronous request
+               if(requestPing != null)
+                       requestPing.Abort(); //cancel an asynchronous request
        }
        public bool Ping(string osVersion, string cjVersion, string machineID) 
        {