Make sure that the value added to the `read_objects_completed` set is
the one we actually return; previously this wasn't the case for conses
because of an optimisation (bug#54501).
Also add a check for vacuous self-references such as #1=#1# instead of
returning a nonsense value from thin air.
* src/lread.c (read1): Treat numbered conses correctly as described
above. Detect vacuous self-references.
* test/src/lread-tests.el (lread-test-read-and-print)
(lread-test-circle-cases, lread-circle): Add tests.
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