imenu fails in mode without comment-start-skip defined

[Dave Love]

I get this when `comment-start-skip' is nil:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  re-search-backward(nil nil t)
  comment-search-backward(nil t)
  comment-beginning()
  imenu--generic-function(((nil "^<<\\(.*\\)>>=$" 1)))
  imenu-default-create-index-function()

due to 

  revision 1.101
  date: 2004/10/26 21:38:09;  author: pfeiffer;  state: Exp;  lines: +29 -22
  (imenu--generic-function): Skip matches in comments.

[I once suggested a different fix for Imenu hits in comments which
didn't fail like this, but it was vetoed.]

I don't think Imenu should require `comment-start-skip' to be defined
(or anything comment-related), but the problem is probably with
newcomment.el.  I'd guess that `comment-beginning' should just return
nil in, say, Fundamental mode.