win32/registry.rb: fix runtime errors

* ext/win32/lib/win32/registry.rb (Win32::Registry::API#EnumKey):
  size of the name is in WCHARs, not in bytes.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43029 b2dd03c8-39d4-4d8f-98ff-823fe69b080e