nfc: fix potential NULL pointer deref in nfc_genl_dump_ses_done

The done() netlink callback nfc_genl_dump_ses_done() should check if
received argument is non-NULL, because its allocation could fail earlier
in dumpit() (nfc_genl_dump_ses()).

Fixes: ac22ac46 ("NFC: Add a GET_SE netlink API")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211209081307.57337-1-krzysztof.kozlowski@canonical.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/nfc/netlink.c

@@ -1394,8 +1394,10 @@ static int nfc_genl_dump_ses_done(struct netlink_callback *cb)
 {
 	struct class_dev_iter *iter = (struct class_dev_iter *) cb->args[0];
 
-	nfc_device_iter_exit(iter);
-	kfree(iter);
+	if (iter) {
+		nfc_device_iter_exit(iter);
+		kfree(iter);
+	}
 
 	return 0;
 }