Don't send bouncer-networks updates for removed networks

We may asynchronously receive an upstream's connection status after
the network has been removed.

Sending an update is pretty bad since it's interpreted by clients
as a new network.

user.go

@@ -910,6 +910,18 @@ func (u *user) handleUpstreamDisconnected(uc *upstreamConn) {
 }
 
 func (u *user) notifyBouncerNetworkState(netID int64, attrs irc.Tags) {
+	// Don't send state updates for removed networks
+	found := false
+	for _, net := range u.networks {
+		if net.ID == netID {
+			found = true
+			break
+		}
+	}
+	if !found {
+		return
+	}
+
 	netIDStr := fmt.Sprintf("%v", netID)
 	for _, dc := range u.downstreamConns {
 		if dc.caps.IsEnabled("soju.im/bouncer-networks-notify") {