PNS: Don't add null items to drag

Not all footprint pad items are connected to elements in the PNS, so if
we don't find the parent element, we avoid later crashes/assertions by
skipping their addition

Fixes #3972 | https://gitlab.com/kicad/code/kicad/issues/3972

pcbnew/router/router_tool.cpp

@@ -1205,12 +1205,16 @@ int ROUTER_TOOL::InlineDrag( const TOOL_EVENT& aEvent )
         for ( const auto p : mod->Pads() )
         {
             auto solid = m_router->GetWorld()->FindItemByParent( p );
+
+            if( solid )
                 itemsToDrag.Add( solid );
         }
     }
     else
     {
         startItem = m_router->GetWorld()->FindItemByParent( static_cast<const BOARD_CONNECTED_ITEM*>( item ) );
+
+        if( startItem)
             itemsToDrag.Add( startItem );
     }