merge revision(s) 54598,54600: [Backport ]

    * thread.c (get_initialized_threadptr): extract ensuring that the
      thread is initialized.

    * thread.c (rb_thread_setname): thread must be initialized to set
      the name.  [ruby-core:74963] [Bug #12290]

    * thread.c (rb_thread_setname): defer setting native thread name
      set in initialize until the native thread is created.
      [ruby-core:74963] [Bug #12290]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@54607 b2dd03c8-39d4-4d8f-98ff-823fe69b080e