cont.c: set th->root_fiber to current fiber at fork

	Otherwise, th->root_fiber can point to an invalid Fiber,
	because Fibers do not live across fork.  So consider
	whatever Fiber is running the root fiber.

	[ruby-core:88723] [Bug #15041]

	cont.c (rb_fiber_atfork): th->root_fiber may not exist

	Otherwise, bootstraptest/test_fork.rb fails with -DVM_CHECK_MODE=2

	[Bug #15041]

	Fixes: r64589 "cont.c: set th->root_fiber to current fiber at fork"

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@66818 b2dd03c8-39d4-4d8f-98ff-823fe69b080e