Use safe math when pruning statuses (#1835)

Co-authored-by: Francisco Aguirre <[email protected]>

Author: KiChjang

cumulus/pallets/xcmp-queue/src/lib.rs

@@ -1129,7 +1129,7 @@ impl<T: Config> XcmpMessageSource for Pallet<T> {
 		let pruned = old_statuses_len - statuses.len();
 		// removing an item from status implies a message being sent, so the result messages must
 		// be no less than the pruned channels.
-		statuses.rotate_left(result.len() - pruned);
+		statuses.rotate_left(result.len().saturating_sub(pruned));
 
 		<OutboundXcmpStatus<T>>::put(statuses);