pallet-scheduler: Unrequest call on failed lookup (#3849)

When the scheduler fails to lookup a `call`, it should unrequest it,
because it will not be required anymore.

substrate/frame/scheduler/src/lib.rs

@@ -1267,6 +1267,17 @@ impl<T: Config> Pallet<T> {
 					id: task.maybe_id,
 				});
 
+				// It was not available when we needed it, so we don't need to have requested it
+				// anymore.
+				T::Preimages::drop(&task.call);
+
+				// We don't know why `peek` failed, thus we most account here for the "full weight".
+				let _ = weight.try_consume(T::WeightInfo::service_task(
+					task.call.lookup_len().map(|x| x as usize),
+					task.maybe_id.is_some(),
+					task.maybe_periodic.is_some(),
+				));
+
 				return Err((Unavailable, Some(task)))
 			},
 		};