Ronnie reported invalid request buffer access in chained command when
inserting garbage value to NextCommand of compound request.
This patch add validation check to avoid this issue.

Cc: Tom Talpey <tom@talpey.com>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Ralph Böhme <slow@samba.org>
Tested-by: NSteve French <smfrench@gmail.com>
Reviewed-by: NSteve French <smfrench@gmail.com>
Acked-by: NHyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: NNamjae Jeon <linkinjeon@kernel.org>
Signed-off-by: NSteve French <stfrench@microsoft.com>