PNS: Safety for evaluating item nets for rules

Fixes https://gitlab.com/kicad/code/kicad/-/issues/13993

pcbnew/pcb_expr_functions.cpp

@@ -896,6 +896,9 @@ static void inDiffPairFunc( LIBEVAL::CONTEXT* aCtx, void* self )
                 {
                     NETINFO_ITEM* netinfo = static_cast<BOARD_CONNECTED_ITEM*>( item )->GetNet();
 
+                    if( !netinfo )
+                        return 0.0;
+
                     wxString refName = netinfo->GetNetname();
                     wxString arg = argv->AsString();
                     wxString baseName, coupledNet;

pcbnew/router/pns_kicad_iface.cpp

@@ -738,6 +738,9 @@ bool PNS_KICAD_IFACE_BASE::ImportSizes( PNS::SIZES_SETTINGS& aSizes, PNS::ITEM*
     int      holeToHoleMin = bds.m_HoleToHoleMin;
     PNS::VIA dummyVia;
 
+    if( aStartItem )
+        dummyVia.SetNet( aStartItem->Net() );
+
     if( m_ruleResolver->QueryConstraint( PNS::CONSTRAINT_TYPE::CT_HOLE_TO_HOLE, &dummyVia,
                                          &dummyVia, UNDEFINED_LAYER, &constraint ) )
     {