Better check for MBE window

[dezull]

The original issue: https://github.com/weynhamz/vim-plugin-minibufexpl/issues/46

This should fix the old issue from fholgado/minibufexpl.vim#119, I don't think it has been resolved yet.

The problem is, at various places this check is performed:

    if bufname('%') != '-MiniBufExplorer-'

to validate the window is indeed MBE's window. Initially that is fine as the bufname matches '-MiniBufExplorer-' exactly.

But after a directory change, it changes to something like:

    /<some-paths...>/.vim_runtime/-MiniBufExplorer-

So this PR performs a better check for MBE's own window.