bug#61890: 30.0.50; Tramp signals an error when completing partial directory names in 'find-file'

[Jim Porter]

Starting from "emacs -Q":

  C-x C-f /ssh:localhost:~/src/emacs/lis/ne/tramp.el <TAB>

This works (it successfully expands "lis/ne" to "lisp/net"), but it 
signals the following error in the minibuffer:

  [File error: tramp-sh-handle-file-name-all-completions: internal 
error accessing ‘/home/user/src/emacs/lis/’: ‘’]

It looks like this is just a sanity check in tramp-sh.el, but I haven't 
had a chance to look any further.