bug#4229: 23.1; wrong-type arg: char or string-p, nil

[Drew Adams]

> > emacs -Q
> > M-x set-variable debug-on-error t
> > M-x load-file /......./bookmark.el
> > C-x r l
> 
> > raises error:
> 
> > Debugger entered--Lisp error: (wrong-type-argument 
> char-or-string-p nil)
> >   insert(nil)
> >   (prog1 (insert (bookmark-location bookmark)) (if (and ... ...)
> 
> I'm afraid I can't reproduce this.  Maybe you need a 
> particular bookmark
> file.  Could you try to make a stand-alone recipe?

Please merge this with bug #4250, which expresses the problem more generally. 

It is a filename entry with nil cdr that causes the error here. Emacs should
simply ignore such a bookmark; it shouldn't cough up a wrong-type-arg error.