storage: check that sideloaded storage is present

If this were passed as nil, we'd be returning "thin" (i.e. with
sideloading payloads not inlined) Entries. This isn't supposed
to happen, but check it.

See:
#31618 (comment).

Release note: None

pkg/storage/replica_raftstorage.go

@@ -84,6 +84,9 @@ func (r *replicaRaftStorage) Entries(lo, hi, maxBytes uint64) ([]raftpb.Entry, e
 	readonly := r.store.Engine().NewReadOnly()
 	defer readonly.Close()
 	ctx := r.AnnotateCtx(context.TODO())
+	if r.raftMu.sideloaded == nil {
+		return nil, errors.New("sideloaded storage is uninitialized")
+	}
 	return entries(ctx, r.mu.stateLoader, readonly, r.RangeID, r.store.raftEntryCache,
 		r.raftMu.sideloaded, lo, hi, maxBytes)
 }