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.
No need in a variable: just `return` this expression. You also missed dedent of closing parenthesis and one newline before this function.

And do not use `vim.current.buffer`, use `cur_buffer` variable from below (on your place I would not bother with additional function at all).