Fix BlockReward contract "arithmetic operation overflow" (#8611)

* Fix BlockReward contract "arithmetic operation overflow"

* Add docs on how execute_as_system works

* Fix typo

ethcore/src/machine.rs

@@ -122,7 +122,13 @@ impl EthereumMachine {
 }
 
 impl EthereumMachine {
-	/// Execute a call as the system address.
+	/// Execute a call as the system address. Block environment information passed to the
+	/// VM is modified to have its gas limit bounded at the upper limit of possible used
+	/// gases including this system call, capped at the maximum value able to be
+	/// represented by U256. This system call modifies the block state, but discards other
+	/// information. If suicides, logs or refunds happen within the system call, they
+	/// will not be executed or recorded. Gas used by this system call will not be counted
+	/// on the block.
 	pub fn execute_as_system(
 		&self,
 		block: &mut ExecutedBlock,
@@ -132,7 +138,7 @@ impl EthereumMachine {
 	) -> Result<Vec<u8>, Error> {
 		let env_info = {
 			let mut env_info = block.env_info();
-			env_info.gas_limit = env_info.gas_used + gas;
+			env_info.gas_limit = env_info.gas_used.saturating_add(gas);
 			env_info
 		};