p2p/enode: remove data race in sliceIter (#20421)

Author: MariusVanDerWijden

p2p/enode/iter.go

@@ -88,6 +88,8 @@ func (it *sliceIter) Next() bool {
 }
 
 func (it *sliceIter) Node() *Node {
+	it.mu.Lock()
+	defer it.mu.Unlock()
 	if len(it.nodes) == 0 {
 		return nil
 	}