bug#62020: Lisp reader: dotted pair notation not working when initial elements are omitted

[Federico Tedin]

According to the Elisp docs
(https://www.gnu.org/software/emacs/manual/html_node/elisp/Dotted-Pair-Notation.html),
one should be able to evaluate e.g.:
    (. 1)
to:
    1

This works correctly in Emacs 28.1 However, in the emacs-29 branch
(bd07cec) this results in:
    *** Read error ***  Invalid read syntax: "."

Since the description in the docs are the same in the emacs-29 I
assume this is a bug.
I believe this may have been caused by the changes implementing the
nonrecursive Lisp reader.
I believe the fix would be roughly:
- in read0 (lread.c), when handling c == '.', ensure we handle not
only the case where the top of the read stack is RE_list but *also*
RE_list_start.
- if the top of the stack was effectively RE_list_start, then the top
of the stack needs to be manipulated somehow so that at the end of
read0 the correct value is returned, when emptying the stack.