Qt4 - fix QT playlist display bug when clicking on top-level of playlist.

Patch by Alexander Gall. Thanks.

modules/gui/qt4/playlist_model.cpp

@@ -744,6 +744,8 @@ void PLModel::rebuild( playlist_item_t *p_root )
     PL_LOCK;
     /* Clear the tree */
     if( rootItem )
+    {
+        if( rootItem->children.size() )
         {
             beginRemoveRows( index( rootItem, 0 ), 0,
                     rootItem->children.size() -1 );
@@ -751,6 +753,7 @@ void PLModel::rebuild( playlist_item_t *p_root )
             rootItem->children.clear();
             endRemoveRows();
         }
+    }
     if( p_root )
     {
         //if( rootItem ) delete rootItem;