After removing items from the conn list, check valid

When cancelling, we mark elements invalid if they are removed from the
connectivity list.  Be cause when iterating through the list again.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20566

pcbnew/connectivity/connectivity_algo.cpp

@@ -945,7 +945,7 @@ void CN_CONNECTIVITY_ALGO::updateJumperPads()
 
     for( CN_ITEM* item : m_itemList )
     {
-        if( item->Parent()->Type() != PCB_PAD_T )
+        if( !item->Valid() || item->Parent()->Type() != PCB_PAD_T )
             continue;
 
         auto pad = static_cast<const PAD*>( item->Parent() );