layout: Fix old event removal

If we increment our index variable while looping, this means that
firstNewEvent will be one higher than it should. With a length 1
array, all events will be removed, so this has a cascading effect
that events will not be stored at all.

https://bugzilla.gnome.org/show_bug.cgi?id=693854

js/ui/layout.js

@@ -1176,8 +1176,9 @@ const PressureBarrier = new Lang.Class({
         // and then chop events after that off.
         let i = 0;
         while (i < this._barrierEvents.length) {
-            if (!this._isBarrierEventTooOld(this._barrierEvents[i++]))
+            if (!this._isBarrierEventTooOld(this._barrierEvents[i]))
                 break;
+            i++;
         }
 
         let firstNewEvent = i;