Before, it would alloc the endpoint fine and then panic later due to out of range.
This ensures it falis at ep alloc time, and with a panic message that says
what's the actual problem: "no free endpoints available".
This restores the Send/Sync impls for EpState, which were incidentally
removed in #2881. This allows the Endpoints to be send to tasks, e.g.
as part of a custom class implementation.
