securerandom.rb: fix substring of FormatMessage result

* lib/securerandom.rb: set the script encoding to make a string
  literal in SecureRandom::Kernel32.last_error_message single byte
  encoding so msg[] works in bytes, since FormatMessage() returns
  the size in TCHARs, not in characters.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48328 b2dd03c8-39d4-4d8f-98ff-823fe69b080e