p2p: reduce the scope of variable dialPubkey (ethereum#24385)

dialPubkey isn't used anywhere else after dialDest.Load, so it should be safe to
restrict its scope to the if clause.

Author: zhiqiangxu

Diff for: p2p/server.go

@@ -943,9 +943,8 @@ func (srv *Server) setupConn(c *conn, flags connFlag, dialDest *enode.Node) erro
 	}
 
 	// If dialing, figure out the remote public key.
-	var dialPubkey *ecdsa.PublicKey
 	if dialDest != nil {
-		dialPubkey = new(ecdsa.PublicKey)
+		dialPubkey := new(ecdsa.PublicKey)
 		if err := dialDest.Load((*enode.Secp256k1)(dialPubkey)); err != nil {
 			err = errors.New("dial destination doesn't have a secp256k1 public key")
 			srv.log.Trace("Setting up connection failed", "addr", c.fd.RemoteAddr(), "conn", c.flags, "err", err)