Lock the playlist so we can add SAP item to the playlist without asserting.

modules/gui/qt4/components/playlist/standardpanel.cpp

@@ -315,11 +315,14 @@ void StandardPLPanel::doPopup( QModelIndex index, QPoint point )
 /* This activated by the selector selection */
 void StandardPLPanel::setRoot( int i_root_id )
 {
+    QPL_LOCK;
     playlist_item_t *p_item = playlist_ItemGetById( THEPL, i_root_id,
                                                     true );
     assert( p_item );
     p_item = playlist_GetPreferredNode( THEPL, p_item );
     assert( p_item );
+    QPL_UNLOCK;
+
     model->rebuild( p_item );
 }