fix(port): failed assert when placing special in overmap editor (#3853)

Fix crash when placing special in overmap editor (#52762)

Co-authored-by: Binrui Dong <[email protected]>

src/overmap.cpp

@@ -1227,7 +1227,12 @@ struct fixed_overmap_special_data : overmap_special_data {
     }
 
     std::vector<overmap_special_terrain> preview_terrains() const override {
-        return terrains;
+        std::vector<overmap_special_terrain> result;
+        std::copy_if( terrains.begin(), terrains.end(), std::back_inserter( result ),
+        []( const overmap_special_terrain & terrain ) {
+            return terrain.p.z == 0;
+        } );
+        return result;
     }
 
     std::vector<overmap_special_locations> required_locations() const override {