search: destroy result actors before forgetting about them

We can't let live (ie, never destroyed) actors undergo GC, because
they will emit :destroy signals during finalization and assert/crash
libmozjs. Properly destroy all actors before letting the GC
free them.

https://bugzilla.gnome.org/show_bug.cgi?id=724798

js/ui/search.js

@@ -315,6 +315,8 @@ const SearchResultsBase = new Lang.Class({
     },
 
     clear: function() {
+        for (let resultId in this._resultDisplays)
+            this._resultDisplays[resultId].actor.destroy();
         this._resultDisplays = {};
         this._clearResultDisplay();
         this.actor.hide();