send_aio() can drop sends if no references to the Aio are kept

[shikokuchuo]

Raised by @wch in posit-dev/acquaint#10.

Reproducible using certain protocols e.g. 'pair'. While one socket is closed, if async sends are performed e.g. for (i in 1:1000) send_aio(s, i) and garbage collection is invoked, those messages are not received when the other party re-connects.