bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> You are suggesting that we go back to using live_string_p?
> I think he's saying just the opposite: namely, that maybe_lisp_pointer is a
> mistake, in that it goes against the (solid) reasons we've replaced some calls
> to live_string_p with calls to live_string_holding.
> After looking into it I agree. I'll propose a patch shortly that does away with
> maybe_lisp_pointer.

Exactly.  More specifically, `maybe_lisp_pointer` tries to filter out
false positives but does it based on the assumption that we should only
accept numbers that look like pointers to the beginning of
a Lisp_Object.

If we still want to try and filter out false positives we need to do it
more carefully by considering what is the smallest alignment possible
for a pointer to an internal field of a Lisp_Object.

And if this least alignment is not the same for all Lisp_Objects, then
this test should likely be moved to the respective `live_<foo>_holding`.

I suspect that for vectorlike objects, the least alignement is 1 because
of some `char` or `bool` fields in some of the pseudovectors.
Of course, we could do better by checking for "false positives" after
checking the specific kind of vectorlike object (so as to use
a different least-alignment-check for those objects that contains
`char`s than for those who only contain `int`s, for example).


        Stefan