Don't crash if scrollbars updated before init

The initialisation here is done on a timer to allow the DOM
elements to be in place before it's run, so it's possible for an
update to happen ebfore the initialisation. Stop & bail out if
there is no this.stickies.

src/components/views/rooms/RoomList.js

@@ -1,6 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
-Copyright 2017 Vector Creations Ltd
+Copyright 2017, 2018 Vector Creations Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -451,6 +451,8 @@ module.exports = React.createClass({
             }
         }
 
+        if (!this.stickies) return;
+
         const self = this;
         let scrollStuckOffset = 0;
         // Scroll to the passed in position, i.e. a header was clicked and in a scroll to state