Don't show buffer if it's not listed

This prevents deleted or hidden buffers to show up
in buffers segment for single tab or current tab.

When working with lot of buffers and closing them on regular basis,
or when buffers where created by plugins (NERDTree for example)
I ended up with lots of entries in buffer line, although :buffers command
didn't show them.
`buffer.options` uses the same code for getting options as uses `setbufvar` (specifically `aucmd_prepbuf` function). This code does some clever things like creating fake windows AFAIK, so it is not impossible to break something. Can you reproduce your problems without powerline, but with `setbufvar()` and if yes file a bug report to vim-dev?

I will check whether I can live without `aucmd_prepbuf` in `OptionsAssItem` (more specifically in `get_option_value_strict`), but AFAIR it was there for a reason.