Fixes use-after-free when accessing the database after it is closed with
`btree_close`. `btree_close` always succeeds when called with an
already-closed database.
The new test checks that operations that access the underlying database
(get, set, flush, seq) fail with a `ValueError` when the btree is already
closed. It also checks that closing and printing the btree succeed when
the btree is already closed.
Fixes issue #12543.
Signed-off-by: Michael Vornovitsky <michaelvornovitskiy@outlook.com>
