iclouddrive: fix potential crash looking up items

findItem read the response status code when the lookup failed, so a
failure with no HTTP response panicked.

Thanks to @manus-pi for finding this problem.

backend/iclouddrive/iclouddrive.go

@@ -92,7 +92,7 @@ func (f *Fs) findItem(ctx context.Context, dir string) (item *api.DriveItem, fou
 		item, resp, err = f.service.GetItemByPath(ctx, path.Join(f.root, dir))
 		return shouldRetry(ctx, resp, err)
 	}); err != nil {
-		if item == nil && resp.StatusCode == 404 {
+		if item == nil && resp != nil && resp.StatusCode == 404 {
 			return nil, false, nil
 		}
 		return nil, false, err