is_directory() wrong with NTFS mounted volumes, and temp_directory_path() fails then

[andreas.malzahn@…]

is_directory() returns false with NTFS mounted volumes in Windows. This look like the same problem as #5649, but the consequences are more serious I think. Some people put the temp directory on a mounted volume, and in this case temp_directory_path() fails although windows has a valid temporary directory. I use some commercial software that uses temp_directory_path() which will fail without useful error messages.