Backwards compatibility with new currency issue and bug 336843.

[Mike Alexander]

--On November 19, 2013 11:38:23 PM +0100 Frédéric Perrin 
<fred at fperrin.net> wrote:

> The attached patch compiles and seems to do what we want from 5
> minutes of testing.

I gave the patch a try and it seems to do the trick.  I tried deleting 
the display symbol slot from all commodities and the commodity itself 
if it contained nothing else, and commodities still display correctly 
and the slots didn't come back when I saved the file.

             Mike