skins2: playlist a few iterator issues

- check validity before using iterators
- don't use iterator anymore once the item is erased

modules/gui/skins2/vars/playtree.cpp

@@ -149,6 +149,7 @@ void Playtree::onUpdateCurrent( bool b_active )
             return;
 
         Iterator it = findById( m_currentItem->i_id );
+        if( it != end() )
             it->m_playing = false;
         m_currentItem = NULL;
     }
@@ -164,6 +165,7 @@ void Playtree::onUpdateCurrent( bool b_active )
         }
 
         Iterator it = findById( current->i_id );
+        if( it != end() )
             it->m_playing = true;
         m_currentItem = current;
 
@@ -187,9 +189,11 @@ void Playtree::onDelete( int i_id )
     Iterator item = findById( i_id ) ;
     if( item != end() )
     {
-        if( item->parent() )
-            item->parent()->removeChild( item );
-        descr.b_visible = item->parent() ? item->parent()->m_expanded : true;
+        VarTree* parent = item->parent();
+        if( parent )
+            parent->removeChild( item );
+
+        descr.b_visible = parent ? parent->m_expanded : true;
         notify( &descr );
     }
 }