Coding warning attributes to wrong char

[Yuchen Pei]

Could reprod in 28.2 and 29.0.91:

1. Open the attached text file, or save the following in a file and open
   it (hopefully displayed correctly here in your email client...)
--8<---------------cut here---------------start------------->8---
   The issue is not with ’, but the  (nul, insert with C-q C-@).
--8<---------------cut here---------------end--------------->8---

2. M-x set-buffer-file-coding-system utf-8 <RET>

3. A warning appears, attributing the issue to the ’, the quote (in the
   following I have replaced the chars with literal strings

--8<---------------cut here---------------start------------->8---
These default coding systems were tried to encode the following
problematic characters in the buffer ‘encoding.txt’:
  Coding System           Pos  Codepoint  Char
  utf-8-unix               23  #x3FFFE2   \342
                           24  #x3FFF80   \200
                           25  #x3FFF99   \231

However, each of them encountered characters it couldn’t encode:
  utf-8-unix cannot encode these: \342 \200 \231

Click on a character (or switch to this window by ‘C-x o’
and select the characters by RET) to jump to the place it appears,
where ‘C-u C-x =’ will give information about it.

Select one of the safe coding systems listed below,
or cancel the writing with C-g and edit the buffer
   to remove or modify the problematic characters,
or specify any other coding system (and risk losing
   the problematic characters).

  raw-text no-conversion
--8<---------------cut here---------------end--------------->8---

Despite the warning, the correct fix is to remove the nul character.

This can be quite misleading, especially when one wants to fix encoding
issues in big text files.

encoding.txt
Description: Text document

Best,
Yuchen

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