Regression in mb_parse_str

In HHVM 3.30.1, the following code gives incorrect results:

```
$a = [];
mb_parse_str("id=&type=foo", $a);
var_dump($a);
```

In HHVM 3.30.1 it outputs:
```
array(1) {
["type"]=>
string(3) "foo"
}
```

However, in php5, php7, and HHVM 3.12.14, it outputs:
```
array(2) {
["id"]=>
string(0) ""
["type"]=>
string(3) "foo"
}
```