Avoid wrapping session index

Bit clearer this way.

substrate/frame/mixnet/src/lib.rs

@@ -290,7 +290,7 @@ pub mod pallet {
 
 			Mixnodes::<T>::insert(
 				// Registering for the _following_ session
-				registration.session_index.wrapping_add(1),
+				registration.session_index + 1,
 				registration.authority_index,
 				registration.mixnode,
 			);
@@ -404,7 +404,13 @@ impl<T: Config> Pallet<T> {
 
 	/// Returns the mixnode set for the previous session.
 	pub fn prev_mixnodes() -> Result<Vec<Mixnode>, MixnodesErr> {
-		Self::mixnodes(CurrentSessionIndex::<T>::get().wrapping_sub(1))
+		let Some(prev_session_index) = CurrentSessionIndex::<T>::get().checked_sub(1) else {
+			return Err(MixnodesErr::InsufficientRegistrations {
+				num: 0,
+				min: T::MinMixnodes::get(),
+			})
+		};
+		Self::mixnodes(prev_session_index)
 	}
 
 	/// Returns the mixnode set for the current session.
@@ -461,7 +467,7 @@ impl<T: Config> Pallet<T> {
 	/// `session_index` should be the index of the current session. `authority_index` is the
 	/// authority index in the _next_ session.
 	fn already_registered(session_index: SessionIndex, authority_index: AuthorityIndex) -> bool {
-		Mixnodes::<T>::contains_key(session_index.wrapping_add(1), authority_index)
+		Mixnodes::<T>::contains_key(session_index + 1, authority_index)
 	}
 
 	/// Try to register a mixnode for the next session.
@@ -566,11 +572,13 @@ impl<T: Config> OneSessionHandler<T::AccountId> for Pallet<T> {
 		CurrentSessionStartBlock::<T>::put(frame_system::Pallet::<T>::block_number());
 
 		// Discard the previous previous mixnode set, which we don't need any more
-		check_removed_all(Mixnodes::<T>::clear_prefix(
-			session_index.wrapping_sub(2),
-			T::MaxAuthorities::get(),
-			None,
-		));
+		if let Some(prev_prev_session_index) = session_index.checked_sub(2) {
+			check_removed_all(Mixnodes::<T>::clear_prefix(
+				prev_prev_session_index,
+				T::MaxAuthorities::get(),
+				None,
+			));
+		}
 
 		if changed {
 			// Save authority set for the next session. Note that we don't care about the authority