Do not panic when changing tiers, and user is nil

server/visitor.go

@@ -330,9 +330,13 @@ func (v *visitor) SetUser(u *user.User) {
 	v.mu.Lock()
 	defer v.mu.Unlock()
 	shouldResetLimiters := v.user.TierID() != u.TierID() // TierID works with nil receiver
-	v.user = u
+	v.user = u                                           // u may be nil!
 	if shouldResetLimiters {
-		v.resetLimitersNoLock(u.Stats.Messages, u.Stats.Emails, true)
+		var messages, emails int64
+		if u != nil {
+			messages, emails = u.Stats.Messages, u.Stats.Emails
+		}
+		v.resetLimitersNoLock(messages, emails, true)
 	}
 }