firmware: fix one UAF issue

There could be the use after free issue in dmi_sysfs_register_handle.
During handling specializations process, the entry->child could be
free if the error occurs. However, it will be kobject_put after free.
So, we set the entry->child to NULL to avoid above case.

Reported-by: loydlv <loydlv@tencent.com>
Signed-off-by: Xinghui Li <korantli@tencent.com>

drivers/firmware/dmi-sysfs.c

@@ -470,6 +470,7 @@ out_del:
 	kobject_del(entry->child);
 out_free:
 	kfree(entry->child);
+	entry->child = NULL;
 	return ret;
 }