extractor: Do not allow symlink in parents
Currently, it is still possible that some files are extracted outside of the destination dir in case of malicious archives. The checks from commit adb067e6 can be still bypassed in certain cases. See file-roller#108 for more details. After some investigation, I am convinced that it would be best to simply disallow symlinks in parents. For example, `tar` fails to extract such files with the `ENOTDIR` error. Let's do the same here. Fixes: #12