efidisk: NULL pointer dereference in is_child()

Function grub_efi_find_last_device() path may return NULL when called
from is_child().

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/disk/efi/efidisk.c

@@ -162,6 +162,9 @@ is_child (struct grub_efidisk_data *child,
     return 0;
 
   ldp = grub_efi_find_last_device_path (dp);
+  if (! ldp)
+    return 0;
+
   ldp->type = GRUB_EFI_END_DEVICE_PATH_TYPE;
   ldp->subtype = GRUB_EFI_END_ENTIRE_DEVICE_PATH_SUBTYPE;
   ldp->length = sizeof (*ldp);