Disallow closing the previous window from `nvim_win_close`, as this will cause
issues.
Again, no telling how safe this is. It also requires exposing old_curwin. :/
Also note that it's possible for the `&cmdheight` to change if, for example,
there are 2 tabpages and `nvim_win_close` is used to close the last window in
the other tabpage while `&stal` is 1. This is addressed in a later commit.