Get rid of a Qt-generated stderr message

The stderr message "Object::disconnect: Unexpected null parameter"
often appeared when closing and opening buffers.
GuiView::on_currentWorkAreaChanged should not pass a null pointer
to Object::disconnect.

src/frontends/qt4/GuiView.cpp

@@ -1022,6 +1022,7 @@ void GuiView::updateWindowTitle(GuiWorkArea * wa)
 
 void GuiView::on_currentWorkAreaChanged(GuiWorkArea * wa)
 {
+	if (d.current_work_area_)
 		QObject::disconnect(d.current_work_area_, SIGNAL(busy(bool)),
 			this, SLOT(setBusy(bool)));
 	disconnectBuffer();