ev_io_start watcher corruption assertion

[mb-m]

Under fairly high load of forwarding (and not easily replicable, unfortunately), spoa-mirror crashed with:

spoa-mirror: ev.c:3885: ev_io_start: Assertion `("libev: ev_io_start called with corrupted watcher", ((WL)w)->next != (WL)w)' failed.

Unfortunately, it's very difficult to tell what the caller was or where this structure failed, apologies, but putting here so there's an issue, and it's visible. It's possible that somewhere in the read/write switching of the spoe protocol handling, that there was a thread-based race (we're running a threaded spoa-mirror), but I really don't know.