If I create a new fiber and the join to himself we get an abort:

 | tarantool> f = require('fiber')
 | ---
 | ...
 |
 | tarantool> f.join(f.self())
 | tarantool: src/fiber.c:407: fiber_join: Assertion `fiber->flags & FIBER_IS_JOINABLE' failed.
 | Aborted (core dumped)

we should better throw an error.