fiber: fix use-after-free on shutdown with lingering fiber join

On Tarantool shutdown we destroy all the fibers in some sequence. We
don't require that all the fibers are finished before shutdown. So it
may turn out that we first destroy some alive fiber and then destroy
another alive fiber which joins the first one. Currently we have
use-after-free issue in this case because clearing `link` field of
the second fiber changes `wake` field of the first fiber.

Close #9406

NO_DOC=bugfix