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

[Stefan Monnier]

> (obj)->contents[1], etc. each time through the loop, and transforms the call
> into foo (*R) as an optimization. When foo calls the garbage collector,
> maybe_lisp_pointer (R) can be false because R doesn't point directly at a Lisp
> object: it points somewhere into the middle of a Lisp object and R's value is
> not GC-aligned.

Indeed, basically `maybe_lisp_pointer` goes against the effort we've put
into replacing `live_string_p` with `live_string_holding` (i.e. to
recognize anything that points into any part of a Lisp_String so as to
prevent collecting it).


        Stefan