currency: equate undefined currency (XXX) to zero value

This is not possible for Regions and Scripts, but it is for
currencies, like base languages.

Change-Id: I3b775fd2bb57881b85b21391ace4c44af98e213c
Reviewed-on: https://go-review.googlesource.com/16945
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
2 files changed