history: Fix bogus return value

We only use lastItem() to reset the history index to the end, so
nobody noticed the utter nonsense in the return value until gjs
started to warn about it. As we don't actually use the value
anywhere, we could just remove it, but the function name implies
that an item is returned, so fix it to behave as advertised.

https://bugzilla.gnome.org/show_bug.cgi?id=781471

js/misc/history.js

@@ -69,7 +69,7 @@ const HistoryManager = new Lang.Class({
             this._indexChanged();
         }
 
-        return this._historyIndex[this._history.length];
+        return this._historyIndex ? this._history[this._historyIndex -1] : null;
     },
 
     addItem: function(input) {