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
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