stringio.c: don't raise after close
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49905 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
stringio.c: don't raise after close
close for consistent to IO#close.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49905 b2dd03c8-39d4-4d8f-98ff-823fe69b080e