ksmbd: fix passing freed memory 'aux_payload_buf'

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>

fs/smb/server/smb2pdu.c

@@ -6312,7 +6312,7 @@ int smb2_read(struct ksmbd_work *work)
 						      aux_payload_buf,
 						      nbytes);
 		kvfree(aux_payload_buf);
-
+		aux_payload_buf = NULL;
 		nbytes = 0;
 		if (remain_bytes < 0) {
 			err = (int)remain_bytes;