Fix lengths in prefix too

cid.go

@@ -266,7 +266,7 @@ func (p Prefix) Sum(data []byte) (*Cid, error) {
 }
 
 func (p Prefix) Bytes() []byte {
-	buf := make([]byte, 16)
+	buf := make([]byte, 4*binary.MaxVarintLen64)
 	n := binary.PutUvarint(buf, p.Version)
 	n += binary.PutUvarint(buf[n:], p.Codec)
 	n += binary.PutUvarint(buf[n:], uint64(p.MhType))

cid_test.go

@@ -124,7 +124,7 @@ func Test16BytesVarint(t *testing.T) {
 	hash, _ := mh.Sum(data, mh.SHA2_256, -1)
 	c := NewCidV1(CBOR, hash)
 
-	c.codec = 1 << 54
+	c.codec = 1 << 63
 	_ = c.Bytes()
 }