Toni discovered a bug where you can open a corrupted archive after opening a valid one. Steps to reproduce:

1. Use right arrow key to enter a valid e.g. valid.7z archive
2. Go back (left arrow key)
3. Create an empty file named "wrong.7z"
4. Use right arrow key to enter this invalid archive -> it fails
5. Do it again -> it succeeds and you are now browsing previous valid archive with "krarc://.../wrong.7z/..." in address bar

This patch should fix it.

• use steps above
• enter valid archive, read files in it, write to it