Fix (pogreb): initialize slice as zero len to avoid empty keys

pogreb/pogreb.go

@@ -23,7 +23,7 @@ func (pstore *Store) Len() int {
 
 func (pstore *Store) Keys() [][]byte {
  iter := pstore.DB.Items()
- ks := make([][]byte, pstore.DB.Count())
+ ks := make([][]byte, 0, pstore.DB.Count())
  for k, _, _ := iter.Next(); k != nil; k, _, _ = iter.Next() {
  ks = append(ks, k)
  }