bug#67683: 30.0.50; [PATCH] Fix 'M-.' on Emacs C sources

[Eli Zaretskii <eliz@gnu.org>]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: 67683@debbugs.gnu.org
> Date: Thu, 18 Jan 2024 22:10:48 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> [...]
> 
> >> I'm not using Edebug but since I currently get "(void-function nil)"
> >> error I did a 'toggle-debug-on-error' and this error triggers when
> >> calling 'tags-included-tables' from 'tags-table-extend-computed-list'
> >> 
> >> What is really weird is that if I set myself into the TAGS buffer and
> >> evaluate (tags-included-tables), it works and returns a list of
> >> filenames.
> >
> > And if you type "C-u M-." in the TAGS buffer, then you can type a
> > symbol, and Xref shows its definition allright?  IOW, does M-. work
> > from the TAGS buffer?
> 
> No "C-u M-." does not work from the TAGS buffer with the same error.

So then the conclusion is that something in the way
tags-included-tables is invoked from tags-table-extend-computed-list
(or its callers in the M-. scenario) breaks tags-included-tables, even
though invoking tags-included-tables directly works as intended,
right?  If so, I suggest to look up the call-stack chain and try to
find what is it that breaks tags-included-tables.