ieee1275: fix signed comparison

grub-core/kern/ieee1275/mmap.c

@@ -49,7 +49,7 @@ grub_machine_mmap_iterate (grub_memory_hook_t hook, void *hook_data)
 					  sizeof available, &available_size))
     return grub_error (GRUB_ERR_UNKNOWN_DEVICE,
 		       "couldn't examine /memory/available property");
-  if (available_size > sizeof (available))
+  if (available_size < 0 || (grub_size_t) available_size > sizeof (available))
     return grub_error (GRUB_ERR_UNKNOWN_DEVICE,
                        "/memory response buffer exceeded");