[PHPBB3-12852] \phpbb\path_helper get_url_parts does not handle get variable with no value

If you do something like :

append_sid($phpbb_root_path . "script.$php_ext", "do=something&continue");

You get a notice :

[phpBB Debug] PHP Notice: in file [ROOT]/phpbb/path_helper.php on line 356: Undefined offset: 1

Because the code assumes that a value is set for the $_REQUEST's 'continue' key :

				list($key, $value) = explode('=', $argument, 2);

I don't think phpbb should enforce such restrictions on urls. Fix could be to wrap this with a strpos test :

				if (strpos($argument, '=') !== false)

					list($key, $value) = explode('=', $argument, 2);

				else

					$key = $argument;

					$value = null;