[pallet-broker] Use saturating math in input validation (#4151)

Changes:
- Saturate in the input validation of he drop history function or
pallet-broker.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/broker/src/dispatchable_impls.rs

@@ -419,7 +419,10 @@ impl<T: Config> Pallet<T> {
 	pub(crate) fn do_drop_history(when: Timeslice) -> DispatchResult {
 		let config = Configuration::<T>::get().ok_or(Error::<T>::Uninitialized)?;
 		let status = Status::<T>::get().ok_or(Error::<T>::Uninitialized)?;
-		ensure!(status.last_timeslice > when + config.contribution_timeout, Error::<T>::StillValid);
+		ensure!(
+			status.last_timeslice > when.saturating_add(config.contribution_timeout),
+			Error::<T>::StillValid
+		);
 		let record = InstaPoolHistory::<T>::take(when).ok_or(Error::<T>::NoHistory)?;
 		if let Some(payout) = record.maybe_payout {
 			let _ = Self::charge(&Self::account_id(), payout);

substrate/frame/broker/src/tests.rs

@@ -146,6 +146,7 @@ fn drop_history_works() {
 			advance_to(16);
 			assert_eq!(InstaPoolHistory::<Test>::iter().count(), 6);
 			advance_to(17);
+			assert_noop!(Broker::do_drop_history(u32::MAX), Error::<Test>::StillValid);
 			assert_noop!(Broker::do_drop_history(region.begin), Error::<Test>::StillValid);
 			advance_to(18);
 			assert_eq!(InstaPoolHistory::<Test>::iter().count(), 6);