[SCSI] QLOGICPTI: Do not unmap DMA unless we actually mapped something.

We only map DMA when cmd->request_bufflen is non-zero for non-sg
buffers, we thus should make the same check when unmapping.

Based upon a report from Pasi Pirhonen.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/scsi/qlogicpti.c

@@ -1281,7 +1281,7 @@ static struct scsi_cmnd *qlogicpti_intr_handler(struct qlogicpti *qpti)
 				      (struct scatterlist *)Cmnd->request_buffer,
 				      Cmnd->use_sg,
 				      Cmnd->sc_data_direction);
-		} else {
+		} else if (Cmnd->request_bufflen) {
 			sbus_unmap_single(qpti->sdev,
 					  (__u32)((unsigned long)Cmnd->SCp.ptr),
 					  Cmnd->request_bufflen,