merge revision(s) 55115: [Backport ]

    * transcode.c (enc_arg, str_transcode_enc_args, econv_args):
      remove volatile, and add GC guards in callers.
      [ruby-core:75664] [Bug #12411]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@55934 b2dd03c8-39d4-4d8f-98ff-823fe69b080e