Ruby » Ruby master

merge revision(s) r45899: [Backport #9751]

    * thread.c (thread_start_func_2): stop if forked in a sub-thread,
      the thread has become the main thread.
      [ruby-core:62070] [Bug #9751]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46497 b2dd03c8-39d4-4d8f-98ff-823fe69b080e