win32/registry.rb: slice in WCHARs

* ext/win32/lib/win32/registry.rb (Win32::Registry::API#Enum{Value,Key):
  ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48333 b2dd03c8-39d4-4d8f-98ff-823fe69b080e