core: fix benchmark panic (ethereum#24657)

This PR fixes a few panics in the chain marker benchmarks. The root
cause for panic is in chain marker the genesis header/block is not
accessible, while it's expected to be obtained in tests. So this PR
avoids touching genesis header at all to avoid panic.

core/bench_test.go

@@ -160,7 +160,7 @@ func genTxRing(naccounts int) func(int, *BlockGen) {
 
 // genUncles generates blocks with two uncle headers.
 func genUncles(i int, gen *BlockGen) {
-	if i >= 6 {
+	if i >= 7 {
 		b2 := gen.PrevBlock(i - 6).Header()
 		b2.Extra = []byte("foo")
 		gen.AddUncle(b2)