bug#65631: Xref updates stack in case of error

[Juri Linkov]

>>>> 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.
>> In case of error, point doesn't move.  So there is no need to go back.
>
> At this point, I was kind of feeling that this is a known-but-tolerated
> behavior (that some might already be relying on, also known as "spacebar
> heater" effect), but if it's still annoying, let's see about a fix.

It's not annoying, just surprises with the requirement to type extra ‘M-,’.

> How's the attached patch?

Thanks.  I expected a smaller change, but if this is still reliable,
then why not.  At least, it works in cases that I tested.