Prevent crash on unplated holes

NPTH may not be flashed on any layer, so the set-layer-visible logic
shouldn't try to determine their layer for showing.

Fixes https://gitlab.com/kicad/code/kicad/issues/11193

pcbnew/dialogs/dialog_drc.cpp

@@ -401,7 +401,7 @@ void DIALOG_DRC::OnDRCItemSelected( wxDataViewEvent& aEvent )
                 LSET layersList = getActiveLayers( it );
                 violationLayers &= layersList;
 
-                if( principalLayer <= UNDEFINED_LAYER )
+                if( principalLayer <= UNDEFINED_LAYER && layersList.count() )
                     principalLayer = layersList.Seq().front();
             }
         }