bug#26317: 26.0.50; Small incompatibility in ibuffer.el (ibuffer-compile-format)

[John Mastro]

In commit 7e2a8b1, ibuffer.el was change to use `string-width' in a couple
places where it previously used `length'. The result is that custom columns
which can return nil, which worked in Emacs 25, will now cause an error.

To reproduce, run `emacs -Q', yank the following into *scratch*, and evaluate
it:

    (require 'ibuffer)

    (define-ibuffer-column my-column
      (:name "X")
      (and (equal (buffer-name) "*scratch*") "Y"))

    (setq ibuffer-formats '((mark modified read-only
                                  " " (name 18 18 :left :elide)
                                  " " (my-column 5 5 :left)
                                  " " filename-and-process)))

    (ibuffer)

It works in Emacs 25, but in Emacs 26 an error is signaled:

    Debugger entered--Lisp error: (wrong-type-argument stringp nil)
      string-width(nil)

I came across this because I use the `ibuffer-vc' package, which defines a
column (`vc-status') whose implementation returns nil for some buffers (by
default, non-file-visiting buffers and buffers visiting remote files).

I will follow up with a patch in a separate email (so I can include the bug
number in the commit message).

        John