destroy prepared geometry in multipolygon builder

geom/geos/prepared.go

@@ -36,3 +36,12 @@ func (this *Geos) PreparedIntersects(a *PreparedGeom, b *Geom) bool {
 	// result == 2 -> exception (already logged to console)
 	return false
 }
+
+func (this *Geos) PreparedDestroy(geom *PreparedGeom) {
+	if geom.v != nil {
+		C.GEOSPreparedGeom_destroy_r(this.v, geom.v)
+		geom.v = nil
+	} else {
+		log.Printf("double free?")
+	}
+}

geom/multipolygon.go

@@ -88,6 +88,9 @@ func BuildRelGeometry(rel *element.Relation, rings []*Ring) (*geos.Geom, error)
 	shells := map[*Ring]bool{rings[0]: true}
 	for i := 0; i < totalRings; i++ {
 		testGeom := g.Prepare(rings[i].geom)
+		if testGeom == nil {
+			return nil, errors.New("Error while preparing geometry")
+		}
 		for j := i + 1; j < totalRings; j++ {
 			if g.PreparedContains(testGeom, rings[j].geom) {
 				if rings[j].containedBy != -1 {
@@ -110,6 +113,7 @@ func BuildRelGeometry(rel *element.Relation, rings []*Ring) (*geos.Geom, error)
 			// add as shell if it is not a hole
 			shells[rings[i]] = true
 		}
+		g.PreparedDestroy(testGeom)
 	}
 
 	relTags := relationTags(rel.Tags, rings[0].ways[0].Tags)