[SCSI] Fix issue reported by coverity in drivers/scsi/scsi_ioctl.c

This patch attempts to fix an issue found in drivers/scsi/scsi_ioctl.c by Coverity.

Error reported:
CID: 3437
Checker: FORWARD_NULL (help)
File: /export2/p4-coverity/mc2/linux26/drivers/scsi/scsi_ioctl.c
Function: scsi_ioctl_send_command
Description: Variable "buf" tracked as NULL was passed to a function that dereferences it.

Patch description:
buf can be NULL if inlen and outlen are both 0. This patch adds check if the
length is non-zero before calling copy from/to user.

Signed-off-by: Jayachandran C. <c.jayachandran@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>