ipn/ipnlocal: return old hwaddrs if missing

If we previously knew of macaddresses of a node, and they
suddenly goes to zero, ignore them and return the previous
hardware addresses.

Updates tailscale/corp#25168

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -360,7 +360,7 @@ func handleC2NPostureIdentityGet(b *LocalBackend, w http.ResponseWriter, r *http
 		// and looks good in client metrics, remove this parameter and always report MAC
 		// addresses.
 		if r.FormValue("hwaddrs") == "true" {
-			res.IfaceHardwareAddrs, err = posture.GetHardwareAddrs()
+			res.IfaceHardwareAddrs, err = b.getHardwareAddrs()
 			if err != nil {
 				b.logf("c2n: GetHardwareAddrs returned error: %v", err)
 			}