re.c: append excape sequence as-is

* re.c (unescape_nonascii): append excape sequence as-is not
  unescaped character, to get rid of unexpected meta-character.
  [ruby-core:67193] [Bug #10670]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49058 b2dd03c8-39d4-4d8f-98ff-823fe69b080e