fs/zfs: Add missing NULL check after grub_strdup() call

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/zfs/zfs.c

@@ -3349,6 +3349,8 @@ dnode_get_fullpath (const char *fullpath, struct subvolume *subvol,
       filename = 0;
       snapname = 0;
       fsname = grub_strdup (fullpath);
+      if (!fsname)
+	return grub_errno;
     }
   else
     {