Fix a warining

grub-core/fs/zfs/zfs.c

@@ -843,6 +843,9 @@ read_device (grub_uint64_t sector, struct grub_zfs_device_desc *desc,
       {
 	grub_err_t err;
 	unsigned i;
+	if (desc->n_children <= 0)
+	  return grub_error (GRUB_ERR_BAD_FS,
+			     "non-positive number of mirror children");
 	for (i = 0; i < desc->n_children; i++)
 	  {
 	    err = read_device (sector, &desc->children[i],