[#XX] policy: Fix IteratorChainsByPrefix method

* If numeric mapping does not exists, then assign id to 0.
* Add check to unit-test.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

policy/policy_contract.go

@@ -112,9 +112,6 @@ func mapKey(kind Kind, name []byte) []byte {
 // a storage key shortening long names. Short entity
 // names are also mapped to prevent collisions in the map.
 func mapToNumeric(ctx storage.Context, kind Kind, name []byte) (mapped int, mappingExists bool) {
-	mapped = 0
-	mappingExists = false
-
 	mKey := mapKey(kind, name)
 	numericID := storage.Get(ctx, mKey)
 	if numericID == nil {
@@ -246,7 +243,12 @@ func ListChainsByPrefix(entity Kind, entityName string, prefix []byte) [][]byte
 
 func IteratorChainsByPrefix(entity Kind, entityName string, prefix []byte) iterator.Iterator {
 	ctx := storage.GetReadOnlyContext()
-	id, _ := mapToNumeric(ctx, entity, []byte(entityName))
+	id, mappingExists := mapToNumeric(ctx, entity, []byte(entityName))
+	if !mappingExists {
+		// All exising numeric mappings starts with 1. So, if id is set to 0, then
+		// iterator will iterate over empty list of chains.
+		id = 0
+	}
 	keyPrefix := storageKey(entity, id, prefix)
 	return storage.Find(ctx, keyPrefix, storage.ValuesOnly)
 }