pcbnew: Do not remove islands from no-net zones

No net zones, by definition, do not connect to any other element.
Therefore, the entire net is an island and should not be removed by the
knockout section

Fixes: lp:1811239
* https://bugs.launchpad.net/kicad/+bug/1811239

pcbnew/zone_filler.cpp

@@ -191,6 +191,10 @@ bool ZONE_FILLER::Fill( std::vector<ZONE_CONTAINER*> aZones, bool aCheck )
 
     for( auto& zone : toFill )
     {
+        // Non-net zones do not have islands by definition
+        if( zone.m_zone->GetNetCode() <= 0 )
+            continue;
+
         std::sort( zone.m_islands.begin(), zone.m_islands.end(), std::greater<int>() );
         SHAPE_POLY_SET poly = zone.m_zone->GetFilledPolysList();