ipn/ipnlocal: fix a nil pointer dereference when serving /localapi/v0/tka/status

Fixes #13330

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/network-lock.go

@@ -538,11 +538,14 @@ func (b *LocalBackend) NetworkLockStatus() *ipnstate.NetworkLockStatus {
 		filtered[i] = b.tka.filtered[i].Clone()
 	}
 
-	visible := make([]*ipnstate.TKAPeer, len(b.netMap.Peers))
+	var visible []*ipnstate.TKAPeer
+	if b.netMap != nil {
+		visible = make([]*ipnstate.TKAPeer, len(b.netMap.Peers))
 		for i, p := range b.netMap.Peers {
 			s := tkaStateFromPeer(p)
 			visible[i] = &s
 		}
+	}
 
 	stateID1, _ := b.tka.authority.StateIDs()