Check for directory protection only on directories in tl_files::protectFolder()

Otherwise a warning will apair:
`Warning: file_exists(): open_basedir restriction in effect. File(/.../files/path/to/file.png/.htaccess) is not within the allowed path(s): (...) in system/modules/core/dca/tl_files.php on line 512`.
I think it make no sense on files ;-)