bug#49723: 28.0.50; Test in coding.c for NUL bytes in filenames is not reliable

[Eli Zaretskii]

> From: Federico Tedin <federicotedin@gmail.com>
> Cc: 49723@debbugs.gnu.org,  Philipp Stephani <phst@google.com>
> Date: Tue, 14 Sep 2021 21:01:16 +0200
> 
> I'm interested in looking into this one since I want to learn more about
> the C side of the codebase. However, I wasn't able to find a call to
> expand-file-name in encode_file_name or encode_file_name_1. I did find
> the null byte check though (CHECK_TYPE + memchr). Maybe I am missing
> something out.

My description was inaccurate: the expand-file-name call usually
precedes the call to ENCODE_FILE, it is not part of encode_file_name.

> I assume that a similar check on expand-file-name should be applied to
> both input arguments, NAME and DEFAULT-DIRECTORY?

I don't think we need that because expand-file-name calls itself on
DEFAULT-DIRECTORY internally.  But we may need to perform the check on
default-directory, if we use it inside expand-file-name.