FileName: Add a soft assertion against invalid parent in parentDir

In case isRoot() returns false, but cleanPath fails to find the parent
directory, return an empty string.

For example: QDir::cleanPath("D:/..") returns "D:/.." on Windows.

I'm not aware of any concrete case when this can happen, but let's be
cautious.

Change-Id: I0f4af5f5c11bb768d99d90290a4a1a6a1bda7c27
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/fileutils.cpp

@@ -631,6 +631,7 @@ FileName FileName::parentDir() const
 
     const QString path = basePath + QLatin1String("/..");
     const QString parent = QDir::cleanPath(path);
+    QTC_ASSERT(parent != path, return FileName());
 
     return FileName::fromString(parent);
 }