Fix bug: connection is already disconnected before idle check on cluster connection pool

[cryeo]

Description

Fix bug that raises ValueError when connection is already disconnected before idle check on cluster connection pool.

https://github.com/NoneGG/aredis/issues/140#issuecomment-826156169

[TheKevJames]

Hi @cryeo -- just wanted to let you know that my org has recently forked aredis as it appears to have become unmaintained and we've merged in this pull request (and all other open PRs against the aredis repo); in case you find it useful, the fork is here and version 2.0.0 is on PyPI with your changes.