ext/standard: Reject null bytes in proc_open() $cwd

[LamentXU123]

In function proc_open, although the we check the parameter $commend to reject NUL bytes, we didn't actually check $cwd, so this would happen:

<?php
  $cwd = sys_get_temp_dir() . '/php-src-proc-open-nul-poc';
  @mkdir($cwd);

  $proc = proc_open(
      [PHP_BINARY, '-r', 'echo getcwd(), PHP_EOL;'],
      [1 => ['pipe', 'w']],
      $pipes,
      $cwd . "\0/ignored"
  );

  echo stream_get_contents($pipes[1]);
  proc_close($proc);

The output would be /php-src-proc-open-nul-poc, ignoring trailing \0/ignored

reproduce in: https://3v4l.org/o56Sq#v

[LamentXU123]

Ah, CI is happy now. Need someone with write access to merge :)