Recursion encoded as NULL

Code:
```shell
$ sapi/cli/php -r '$a=[&$a]; $b = msgpack_unpack(msgpack_pack($a)); var_dump($b);'
```
Expected output:
```
array(1) {
  [0]=>
  &array(1) {
    [0]=>
    *RECURSION*
  }
}
```
Actual output:
```
array(1) {
  [0]=>
  array(1) {
    [0]=>
    NULL
  }
}
```
