mixclient: Avoid jitter calculation panic

rand.Duration may not be called with a negative or zero upper bound, but this
was seen to occur in (*Client).prDelay().  Two notable possible bugs stood out.

First, if sendBefore is exactly equal to now, then it will not be incremented
by another epoch duration, leading to a potential invalid rand.Duration
parameter.  This is corrected by also checking for the times equaling exactly.

Second, time.Until() causes an additional call to time.Now(), which we have
already fetched and all calculations must be based on it.  If
sendBefore.Sub(now) is an extremely small value, it is possible that
time.Until(sendBefore) now returns a small negative or zero duration.  This is
corrected by replacing the time.Until call with sendBefore.Sub(now).

mixing/mixclient/client.go

@@ -516,11 +516,11 @@ func (c *Client) prDelay(ctx context.Context, p *peer) error {
 	sendBefore := epoch.Add(-timeoutDuration - maxJitter)
 	sendAfter := epoch.Add(timeoutDuration)
 	var wait time.Duration
-	if now.After(sendBefore) {
+	if now.Equal(sendBefore) || now.After(sendBefore) {
 		wait = sendAfter.Sub(now)
 		sendBefore = sendBefore.Add(c.epoch)
 	}
-	wait += p.msgJitter() + rand.Duration(time.Until(sendBefore))
+	wait += p.msgJitter() + rand.Duration(sendBefore.Sub(now))
 	timer := time.NewTimer(wait)
 	select {
 	case <-ctx.Done():