bug#45540: 27.1; symbolic links crash dired

[Devon Sean McCullough]

Here's a fix.  I did NOT check this carefully.
Dired crashes when (< (length file-name) (length root))

diff -aBbdu --label 
/Applications/Emacs-27.1.app/Contents/Resources/lisp/files.el.gz --label 
\#\<buffer\ files.el.gz\> 
/var/folders/jv/9lctn91j0xqfv3f2kxc1lxx80000gn/T/jka-comtv7p5N 
/var/folders/jv/9lctn91j0xqfv3f2kxc1lxx80000gn/T/buffer-content-fatCkh
--- /Applications/Emacs-27.1.app/Contents/Resources/lisp/files.el.gz
+++ #<buffer files.el.gz>
@@ -4019,10 +4019,9 @@
   (let* ((file-name (or (buffer-file-name)
                        ;; Handle non-file buffers, too.
                        (expand-file-name default-directory)))
-        (sub-file-name (if (and file-name
+        (sub-file-name (when (and file-name
                                  (file-name-absolute-p file-name))
-                            ;; FIXME: Why not use file-relative-name?
-                           (substring file-name (length root)))))
+                         (file-relative-name file-name root))))
     (condition-case err
         (dolist (entry class-variables variables)
           (let ((key (car entry)))

Diff finished.  Tue Dec 29 16:42:36 2020