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