Do not have postcode node appear in addresses directly

Many of the postcode nodes are actually derived from
incomplete addresses and are as such not even centroids.
Better let them only take part in the address computation
via the postcode table.

sql/functions.sql

@@ -864,6 +864,10 @@ BEGIN
         SELECT * FROM get_postcode_rank(NEW.country_code, NEW.address->'postcode')
           INTO NEW.rank_search, NEW.rank_address;
 
+        IF NOT ST_GeometryType(NEW.geometry) IN ('ST_Polygon','ST_MultiPolygon') THEN
+            NEW.rank_address := 0;
+        END IF;
+
     ELSEIF NEW.class = 'place' THEN
       IF NEW.type in ('continent') THEN
         NEW.rank_search := 2;