Summary:
Fixes a bug introduced in https://github.com/facebook/rocksdb/issues/9526 where we index one position past the
end of a `vector`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9542

Test Plan:
`make asan_check`

Will add a unit test in a separate PR.

Reviewed By: akankshamahajan15

Differential Revision: D34145825

Pulled By: ltamasi

fbshipit-source-id: 4e87c948407dee489d669a3e41f59e2fcc1228d8