Fix invariant violation.

The comments for msgReleaser suggest we can't hold the lock while doing
this, so add it to the releaseList instead. I'd like to make this harder
to mess up in the future.

rpc/answer.go

@@ -268,7 +268,7 @@ func (ans *ansent) prepareSendReturn(c *lockedConn, rl *releaseList) {
 	select {
 	case <-c.bgctx.Done():
 		// We're not going to send the message after all, so don't forget to release it.
-		ans.returner.msgReleaser.Decr()
+		rl.Add(ans.returner.msgReleaser.Decr)
 		ans.sendMsg = nil
 	default:
 	}