Txt conversion => azw3 failed

[rxliuli]

## Surroundings

- System: Windows 10
- Version: 5.23.0 portable version

## Error

```sh
PS D:\> ebook-convert.exe .\反转凉宫之虚子.txt .\反转凉宫之虚子.azw3
1% 将输入转换为HTML中...
InputFormatPlugin: TXT Input running
on D:\反转凉宫之虚子.txt
Traceback (most recent call last):
  File "runpy.py", line 194, in _run_module_as_main
  File "runpy.py", line 87, in _run_code
  File "site.py", line 82, in <module>
  File "site.py", line 77, in main
  File "site.py", line 49, in run_entry_point
  File "calibre\ebooks\conversion\cli.py", line 401, in main
  File "calibre\ebooks\conversion\plumber.py", line 1105, in run
  File "calibre\customize\conversion.py", line 245, in __call__
  File "calibre\ebooks\conversion\plugins\txt_input.py", line 289, in convert
  File "calibre\ebooks\conversion\plugins\txt_input.py", line 120, in fix_resources
FileNotFoundError: [Errno 2] No such file or directory: 'D:\\'
```

I added the incorrectly converted txt file to the attachment, hoping to fix it or give a clearer error message

[Kovid Goyal (kovid)]

Fixed in branch master. The fix will be in the next release. calibre is usually released every alternate Friday.

 status fixreleased