Don't re-tint on unmount

Need to remember the 'ref' function is called on unmount too - we shouldn't
retint back to the room colours at this point.

src/components/structures/RoomView.js

@@ -1259,7 +1259,9 @@ module.exports = React.createClass({
         var messagePanel = (
             <TimelinePanel ref={(r) => {
                     this.refs.messagePanel = r;
+                    if(r) {
                         this.updateTint();
+                    }
                 }}
                 room={this.state.room}
                 hidden={hideMessagePanel}