Fix panics in debug_TraceTransaction (ethereum#27)

* Make sure L1CostFunc is set on returned context

* Fix panic if HistoricalRPCService is not set

* Can we just return nil?

* Simplify

eth/state_accessor.go

@@ -208,10 +208,10 @@ func (eth *Ethereum) stateAtTransaction(block *types.Block, txIndex int, reexec
 		msg, _ := tx.AsMessage(signer, block.BaseFee())
 		txContext := core.NewEVMTxContext(msg)
 		context := core.NewEVMBlockContext(block.Header(), eth.blockchain, nil)
+		context.L1CostFunc = types.NewL1CostFunc(eth.blockchain.Config(), statedb)
 		if idx == txIndex {
 			return msg, context, statedb, release, nil
 		}
-		context.L1CostFunc = types.NewL1CostFunc(eth.blockchain.Config(), statedb)
 		// Not yet the searched for transaction, execute on top of the current state
 		vmenv := vm.NewEVM(context, txContext, statedb, eth.blockchain.Config(), vm.Config{})
 		statedb.Prepare(tx.Hash(), idx)

eth/tracers/api.go

@@ -827,7 +827,7 @@ func (api *API) TraceTransaction(ctx context.Context, hash common.Hash, config *
 	if err != nil {
 		return nil, err
 	}
-	if tx == nil {
+	if tx == nil && api.backend.HistoricalRPCService() != nil {
 		var histResult []*txTraceResult
 		err = api.backend.HistoricalRPCService().CallContext(ctx, &histResult, "debug_traceTransaction", hash, config)
 		if err != nil && err.Error() == "not found" {