ext/phar: improve .phar madic directory preservation logic in phar::addEmptyDir()

[LamentXU123]

Now, the .phar directory is a magic dir for phar files, and in phar::addEmptyDir(), users couldn't create a dir naming .phar

The implementation is:

	if (zend_string_starts_with_literal(dir_name, ".phar")) {
		zend_throw_exception_ex(spl_ce_BadMethodCallException, 0, "Cannot create a directory in magic \".phar\" directory");
		RETURN_THROWS();

This has two bugs.

Firstly, people can use /.phar to create the .phar dir. The leading / will be ignored. (no need to concern about ../ though, it will be ignored.)

<?php
  $phar = new Phar(__DIR__ . '/test.phar', 0, 'test.phar');
  $phar->addEmptyDir('/.phar');
  var_dump(is_dir('phar://' . __DIR__ . '/test.phar/.phar'));

Will return true, while if the dir is .phar it will raise an error.

Secondly, it only matches the prefix. That said, /.pharxxx will not be allowed to create, which is not a magic dir.

<?php
  $phar = new Phar(__DIR__ . '/test.phar', 0, 'test.phar');
  $phar->addEmptyDir('.pharx');

This will raise an error.

PHP Fatal error:  Uncaught BadMethodCallException: Cannot create a directory in magic ".phar" directory in C:\Users\admin\Desktop\bench.php:3

This PR fix both by 1. adding a trailing check of the path to make .pharx valid 2. adding a check to /.phar

cc @Girgias Thanks!

[LamentXU123]

Not sure about the \0 logic here.