Description:
------------
When trying to use param_handling to call a simple function to check a link, it crashes and the Apache-log says, that a segmentation fault occured. It only happens in 1.0.3b1, in 1.0.2 it's fine.

Reproduce code:
---------------
function checkLink($content, $argument) {
  if(substr($content, 0, 7)) != 'http://') return 'http://'.$content;
  else return $content;
}
$pd['url']['type'] = BBCODE_TYPE_OPTARG;
$pd['url']['open_tag'] = '<a href="{PARAM}">';
$pd['url']['close_tag'] = '</a>';
$pd['url']['default_arg'] = '{CONTENT}';
$pd['url']['param_handling'] = 'checkLink';
$parser = bbcode_create($pd);
echo bbcode_parse($parser, '[url]php.net[/url]');

Expected result:
----------------
<a href="http://php.net">php.net</a>

Actual result:
--------------
crash