messageTray: Reset clickedSummaryItem on ungrab

If we don't, we will pop up the summary again the next time
_updateState() is called.

https://bugzilla.gnome.org/show_bug.cgi?id=707600

js/ui/messageTray.js

@@ -2732,6 +2732,7 @@ const MessageTray = new Lang.Class({
 
     _onSummaryBoxPointerUngrabbed: function() {
         this._summaryBoxPointerState = State.HIDING;
+        this._setClickedSummaryItem(null);
 
         if (this._summaryBoxPointerContentUpdatedId) {
             this._summaryBoxPointerItem.disconnect(this._summaryBoxPointerContentUpdatedId);