scsi_debug: schedule_resp fix input variable check

The function should never be called with cmnd NULL so
put a fat WARN there.
Fix also smatch wraning:
schedule_resp() warn: variable dereferenced before check 'cmnd'

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>