Delete quickfix/loclist to empty will cause ferret#private#qf_delete error

Reproduction steps:
1. Create a qflist or loclist manually or by any plugins.
    e.g., `call setqflist([{'text':'aaa','type':'w'}], 'a')` and then `copen 1`
2. Delete the item by `dd`
3. It will show an error:
    <img width="658" alt="image" src="https://user-images.githubusercontent.com/1425636/163710929-3c1d747d-a6f2-4190-a608-8866735830b8.png">
