fix possible infinite loop in intro.cpp thread

- it was possible to trigger an infinite loop in FreespaceChecker::check() by
  simply removing the drive letter on Windows (which leads to an infinite
  loop in the FreespaceChecker thread)
- this was caused by not checking if we make progress with
  parentDir.parent_path()
Merge pull request #2850 from Diapolo/fix_intro

fix possible infinite loop in intro.cpp thread