p2p/discover: swap verification order in discv4 ping handler (#27532)

In all other UDPv4 methods, the deadline is checked first. It seems weird to me that ping is an exception. Deadline comparison is also less resource intensive.
Co-authored-by: Exca-DK <Exca-DK@users.noreply.github.com>

p2p/discover/v4_udp.go

@@ -643,13 +643,13 @@ type packetHandlerV4 struct {
 func (t *UDPv4) verifyPing(h *packetHandlerV4, from *net.UDPAddr, fromID enode.ID, fromKey v4wire.Pubkey) error {
 	req := h.Packet.(*v4wire.Ping)
 
+	if v4wire.Expired(req.Expiration) {
+		return errExpired
+	}
 	senderKey, err := v4wire.DecodePubkey(crypto.S256(), fromKey)
 	if err != nil {
 		return err
 	}
-	if v4wire.Expired(req.Expiration) {
-		return errExpired
-	}
 	h.senderKey = senderKey
 	return nil
 }