Make sure showDotCursor can be modified before connecting

The cursor object is only attached to our canvas whilst connecting,
so we need to make sure we don't try to update anything when were
not connected or we'll get a crash.

core/rfb.js

@@ -1699,6 +1699,7 @@ export default class RFB extends EventTargetMixin {
     }
 
     _refreshCursor() {
+        if (this._rfb_connection_state !== 'connected') { return; }
         const image = this._shouldShowDotCursor() ? RFB.cursors.dot : this._cursorImage;
         this._cursor.change(image.rgbaPixels,
                             image.hotx, image.hoty,