endSessionDialog: Fix a warning

If _timerId is undefined/null, as it is by default, we will take this
path, and fail when trying to remove a source ID for undefined.

https://bugzilla.gnome.org/show_bug.cgi?id=706612

js/ui/endSessionDialog.js

@@ -442,7 +442,7 @@ const EndSessionDialog = new Lang.Class({
     },
 
     _stopTimer: function() {
-        if (this._timerId != 0) {
+        if (this._timerId > 0) {
             Mainloop.source_remove(this._timerId);
             this._timerId = 0;
         }