bug#65631: Xref updates stack in case of error

[Eli Zaretskii]

> Cc: dmitry gutov <dmitry@gutov.dev>
> From: Juri Linkov <juri@linkov.net>
> Date: Wed, 30 Aug 2023 19:44:13 +0300
> 
> X-Debbugs-CC: Dmitry Gutov <dmitry@gutov.dev>
> 
> 0. emacs -Q
> 1. move point to any word
> 2. type 'M-.' (xref-find-definitions)
> 3. an error is displayed correctly "No definitions found for: This"
> 
> But the problem is that an unrecognized word is added to the xref stack.
> So the context menu shows "Go Back", and 'M-,' (xref-go-back) goes back
> to that word.

FWIW, I'm not sure this is necessarily a bug.  It looks like a bug in
your case, because you deliberately tricked Xref into using a word
that is definitely not a program symbol.  But that is not the case in
a more reasonable situation, where point is on a symbol, but for some
reason the symbol's definition is not found, e.g., because the TAGS
table needs to be regenerated.