fixes EIP1559 tx gasCost validation

pre EIP1559 max(gasCost) is tx.gasLimit * tx.gasPrice
the new EIP1559 max(gasCost) before the transaction can be executed is
tx.gasLimit * tx.maxFeePerGas

nimbus/p2p/validate.nim

@@ -304,7 +304,12 @@ proc validateTransaction*(vmState: BaseVMState, tx: Transaction,
       maxPriorityFee=tx.maxPriorityFee
     return
 
-  let gasCost = tx.gasLimit.u256 * tx.gasPrice.u256
+  # the signer must be able to afford the transaction
+  let gasCost = if tx.txType >= TxEip1559:
+                  tx.gasLimit.u256 * tx.maxFee.u256
+                else:
+                  tx.gasLimit.u256 * tx.gasPrice.u256
+
   if gasCost > balance:
     debug "invalid tx: not enough cash for gas",
       available=balance,