kern: Check for NULL when closing devices and disks

Add checks for NULL pointers to grub_device_close() and
grub_disk_close() to make these functions more robust.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/kern/device.c

@@ -71,6 +71,9 @@ grub_device_open (const char *name)
 grub_err_t
 grub_device_close (grub_device_t device)
 {
+  if (device == NULL)
+    return GRUB_ERR_NONE;
+
   if (device->disk)
     grub_disk_close (device->disk);
 

grub-core/kern/disk.c

@@ -292,6 +292,10 @@ void
 grub_disk_close (grub_disk_t disk)
 {
   grub_partition_t part;
+
+  if (disk == NULL)
+    return;
+
   grub_dprintf ("disk", "Closing `%s'.\n", disk->name);
 
   if (disk->dev && disk->dev->disk_close)