ksmbd: fix passing freed memory 'aux_payload_buf'

[ Upstream commit 59d8d24f ]

The patch e2b76ab8: "ksmbd: add support for read compound" leads
to the following Smatch static checker warning:

  fs/smb/server/smb2pdu.c:6329 smb2_read()
        warn: passing freed memory 'aux_payload_buf'

It doesn't matter that we're passing a freed variable because nbytes is
zero. This patch set "aux_payload_buf = NULL" to make smatch silence.

Fixes: e2b76ab8 ("ksmbd: add support for read compound")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>