fix: remove invalid_tabs before _for_each_state

[hrsh7th]

fix #1067

problem

Probably, neovim does not fire the TabClosed autocmd in some conditions.
So invalid tabs might be remained to the manager states.

solution

check & dispose invalid states before enumerate it.

[cseickel]

I actually came here OK to put an "unmaintained notice" on all PRs, this one is too obvious not to merge.

If you have any interest in maintaining another plugin, neo-tree is up for grabs.