scsi: qedf: Add additional checks for io_req->sc_cmd validity

 - Check the validity of various pointers before processing.

Signed-off-by: Chad Dupuis <cdupuis@marvell.com>
Signed-off-by: Saurav Kashyap <skashyap@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>