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.cpp

@@ -25,6 +25,7 @@ static const uint64 BLOCK_CHAIN_SIZE = 10LL * GB_BYTES;
 class FreespaceChecker : public QObject
 {
     Q_OBJECT
+
 public:
     FreespaceChecker(Intro *intro);
 
@@ -61,9 +62,16 @@ void FreespaceChecker::check()
 
     /* Find first parent that exists, so that fs::space does not fail */
     fs::path parentDir = dataDir;
+    fs::path parentDirOld = fs::path();
     while(parentDir.has_parent_path() && !fs::exists(parentDir))
     {
         parentDir = parentDir.parent_path();
+
+        /* Check if we make any progress, break if not to prevent an infinite loop here */
+        if (parentDirOld == parentDir)
+            break;
+
+        parentDirOld = parentDir;
     }
 
     try {

src/qt/intro.h

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