dash: destroy items's child before tooltip

Destroy the DashItemContainer's child from the same handler as the tooltip. This
will prevent invalid reads when the item is destroyed while its quicklist is
still open.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/781

(cherry picked from commit ff2fbf5ae40561b403b721e932d1bc5492532156)

js/ui/dash.js

@@ -52,6 +52,8 @@ var DashItemContainer = new Lang.Class({
         this.animatingOut = false;
 
         this.connect('destroy', () => {
+            if (this.child != null)
+                this.child.destroy();
             this.label.destroy();
         });
     },