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()

src/qt/intro.h

@@ -37,6 +37,7 @@ public:
      * Determine default data directory for operating system.
      */
     static QString getDefaultDataDirectory();
+
 signals:
     void requestCheck();
     void stopThread();