bug#60467: 30.0.50; primitive-undo: Changes to be undone by function different from announced

[Gregory Heytings]

> > > I don't even understand what this is supposed to do.
> >
> > Yet you happily threw it away :-(
>
> Because there are no such precautions elsewhere in the code, and the 
> comment above ("In case garbage collection has removed OLD-BUL") does 
> not explain what its purpose is.  A few lines below, old-bul is used 
> without such a precaution.  Of course, if it has a purpose, it's okay to 
> keep it.

I didn't remember the main reason why I removed that (cdr ptr).  The 
reason is not only that the comment above it is wrong, but also that this 
(cdr ptr) condition itself is wrong: it makes that loop stop when the last 
element of the buffer-undo-list is reached.  When buffer-undo-list is 
initially nil, (funcall body) adds a number of entries in 
buffer-undo-list, and there is no reason to exit that loop without 
processing its last element.