verifiers: Fix calling uninitialized function pointer

The necessary check for NULL before use of function ver->close is not
taking place in the failure path. This patch simply adds the missing
check and fixes the problem that GRUB hangs indefinitely after booting
rogue image without valid signature if secure boot is turned on.

Now it displays like this for booting rogue UEFI image:

  error: bad shim signature
  error: you need to load the kernel first

  Press any key to continue...

and then you can go back to boot menu by pressing any key or after a few
seconds expired.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/verifiers.c

@@ -196,7 +196,8 @@ grub_verifiers_open (grub_file_t io, enum grub_file_type type)
   return ret;
 
  fail:
-  ver->close (context);
+  if (ver->close)
+    ver->close (context);
  fail_noclose:
   verified_free (verified);
   grub_free (ret);