ksmbd: check iov vector index in ksmbd_conn_write()

If ->iov_idx is zero, This means that the iov vector for the response
was not added during the request process. In other words, it means that
there is a problem in generating a response, So this patch return as
an error to avoid NULL pointer dereferencing problem.
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/connection.c

@@ -197,6 +197,9 @@ int ksmbd_conn_write(struct ksmbd_work *work)
 	if (work->send_no_response)
 		return 0;
 
+	if (!work->iov_idx)
+		return -EINVAL;
+
 	ksmbd_conn_lock(conn);
 	sent = conn->transport->ops->writev(conn->transport, work->iov,
 			work->iov_cnt,