core: verify genesis extradata for clique (ethereum#24470)

* Add extra-data checks for clique genesis

* Update genesis.go

* Update genesis.go

* core: simplify clique genesis check

Co-authored-by: Felix Lange <fjl@twurst.com>
Co-authored-by: Martin Holst Swende <martin@swende.se>

core/genesis.go

@@ -404,7 +404,7 @@ func (g *Genesis) Commit(db ethdb.Database) (*types.Block, error) {
 	if err := config.CheckConfigForkOrder(); err != nil {
 		return nil, err
 	}
-	if config.Clique != nil && len(block.Extra()) == 0 {
+	if config.Clique != nil && len(block.Extra()) < 32+crypto.SignatureLength {
 		return nil, errors.New("can't start clique chain without signers")
 	}
 	if err := g.Alloc.write(db, block.Hash()); err != nil {