Manually check pad layers before inserting

We do not rely on GetEffectiveShape(layer) to return null for layers
that the pad does not exist on.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21523

(cherry picked from commit 2e1910ca86)

pcbnew/drc/drc_creepage_utils.cpp

@@ -2560,7 +2560,7 @@ std::shared_ptr<GRAPH_NODE> CREEPAGE_GRAPH::AddNetElements( int aNetCode, PCB_LA
     {
         for( PAD* pad : footprint->Pads() )
         {
-            if( pad->GetNetCode() != aNetCode )
+            if( pad->GetNetCode() != aNetCode || !pad->GetLayerSet().test( aLayer ) )
                 continue;
 
             std::shared_ptr<SHAPE> padShape = pad->GetEffectiveShape( aLayer );