bug#18337: 24.4.50; apropos fails on "scroll"

[Stefan Monnier]

> (when (setq properties (symbol-plist 'vertical-scroll-bar))
>   (setq doc (list (car properties)))
>   (while (setq properties (cdr (cdr properties)))
>     (setq doc (cons (car properties) doc)))
>   (nreverse doc))

> => (0 event-symbol-element-mask event-symbol-elements modifier-cache 
> variable-documentation)
  
> Applying 'symbol-name to that list then yields the error.

Note that, while convention is to use a symbol for property names, that
is not the only valid choice.  0 is a valid property name (tho an odd
one, arguably).

So while this 0 should probably be changed the mapconcat code that burps
on it should probably be changed not to assume that only symbols will
be used as property names.


        Stefan