[SCSI] aacraid: check buffer address in aac_internal_transfer

Captured a panic on an older kernel where an application issuing
commands via sg was sending requests that lacked a request_buffer, thus
the buffer pointer used in aac_internal_transer was NULL. The
application was fixed closing the issue, but felt it was advised to
immunize the driver against the eventuality.
Signed-off-by: Mark Salyzyn <aacraid@adaptec.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/aacraid/aachba.c

@@ -350,8 +350,9 @@ static void aac_internal_transfer(struct scsi_cmnd *scsicmd, void *data, unsigne
 		buf = scsicmd->request_buffer;
 		transfer_len = min(scsicmd->request_bufflen, len + offset);
 	}
-
-	memcpy(buf + offset, data, transfer_len - offset);
+	transfer_len -= offset;
+	if (buf && transfer_len)
+		memcpy(buf + offset, data, transfer_len);
 
 	if (scsicmd->use_sg) 
 		kunmap_atomic(buf - sg->offset, KM_IRQ0);