stringio.c: non-ascii encoding

* ext/stringio/stringio.c (strio_putc): fix for non-ascii
  encoding, like as IO#putc.  [ruby-dev:48114] [Bug #9765]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45646 b2dd03c8-39d4-4d8f-98ff-823fe69b080e