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: phst@google.com,  49723@debbugs.gnu.org
> Date: Wed, 15 Sep 2021 00:17:33 +0200
> 
> Ok! Here's my first try at this. I ended up skipping the check on
> DEFAULT-DIRECTORY since as you mentioned, its value is used with
> expand-file-name itself. In the other case, if default-directory is
> picked up, then I checked the value of that variable.

Thanks, this LGTM.

> +** 'expand-file-name' now checks for null bytes in filenames.
> +The function will now check for null bytes in both NAME and
> +DEFAULT-DIRECTORY arguments, as well as in the 'default-directory'
> +buffer-local variable, assuming its value is used.

This should say that if null bytes are found, expand-file-name will
signal an error.