jsonb numerics are treated as arrays

Hey!

A short transcript is better than a thousand words: on PostgreSQL 16.1,
I'm getting:

```
postgres=> SELECT ('42'::jsonb)->0, ('42'::jsonb)->(-1);
 ?column? | ?column?
----------+----------
 42       | 42
(1 row)
```

Dereferencing them with `1` or `-2` returns null as expected, indicating
that jsonb numerics are currently being handled as one-element arrays.

Considering the documentation[1] and especially the note there, I'd have
expected numerics dereferencing to result in `NULL`, because they are
not actually arrays.

[1] https://www.postgresql.org/docs/current/functions-json.html#FUNCTIONS-JSON-PROCESSING

TBH, considering this behavior has been present since PostgreSQL 14 at
least, I'd guess it's probably something that can no longer change and
will need to just be documented as being the current behavior.

But I still wanted to raise this on the bugs mailing list, before
sending it to the docs mailing list, because maybe it's behavior that
could still be subject to change?

I'll let you do as you wish, and will work around the issue myself by
checking for jsonb_typeof before dereferencing with 0 or -1 :)

Anyway, thank you for all you do on postgresql, it's a real pleasure to
use! :D
  Léo

PS: FWIW, this issue was discovered by the fuzzers for my library when I
left them running overnight, comparing the postgresql backend with a
"completely in-memory" equivalent model.