Fix preg_match returning false instead of null

`preg_match`/`preg_match_all` should return `NULL` not `FALSE` when invalid flags are passed.

Example:

```php
<?php
var_dump(preg_match('/test/', 'Hello World', $dummy, 0xdead));
```

HHVM Output:

```
HipHop Warning: Invalid flags specified in /var/share/dev/hhvm/hphp/test/slow/ext_preg/preg_match_invalid_flags.php on line 2
false
```

Correct Output:

```
HipHop Warning: Invalid flags specified in /var/share/dev/hhvm/hphp/test/slow/ext_preg/preg_match_invalid_flags.php on line 2
NULL
```