bug#71727: Deleting TAGS buffer will cause `etags-regen--update-file` doesn't work

[Eli Zaretskii <eliz@gnu.org>]

> From: Siyuan Chen <chansey97@gmail.com>
> Date: Sun, 23 Jun 2024 07:55:49 +0800
> 
> Reproduce steps:
> 
> 1. init.el
> ```
> (setq etags-regen-program "C:/green/emacs/emacs-29.3/bin/etags.exe")
> (require 'etags-regen)
> ```
> 
> 2. Open test.c with the following code
> ```
> #define APPLICATION_WINDOW_WIDTH 320
> ```
> 
> 3. M-x etags-regen-mode
> 
> 4. Move the cursor to APPLICATION_WINDOW_WIDTH and M-x xref-find-definitions. Then the TAGS file is
> created and the APPLICATION_WINDOW_WIDTH line is highlighted (good).
> 
> 5. M-x kill-buffer TAGS
> 
> 6. Add `#define APPLICATION_WINDOW_HEIGHT 320` to test.c and M-x save-buffer
> 
> 7. Move the cursor to APPLICATION_WINDOW_HEIGHT and M-x xref-find-definitions.
> 
> The expected behavior: The TAGS file is updated and the APPLICATION_WINDOW_HEIGHT line is
> highlighted.
> 
> The actual behavior: The echo area reports: "No definition found for: APPLICATION_WINDOW_HEIGHT"
> 
> Note that if I don't do step-5 then everything is OK.
> 
> Emacs 29.3 has no this feature, I make it available via copy & paste etags-regen.el to load-path. I think this
> issue has nothing to do with the Emacs version though. 

Thanks.

Dmitry, could you please look into this?