[RFC]Fixed sign column redraw

It should redraw if the buffer is NULL not otherwise
Vim upstream still checks `!= NULL`: https://code.google.com/p/vim/source/browse/src/buffer.c#5710

This is a bit confusing though, because `buf_delsign()` checks `== NULL`:

```c
    if (buf->b_signlist == NULL)
    {
       redraw_buf_later(buf, NOT_VALID);
       changed_cline_bef_curs();
    }
```