st: fix potential null pointer dereference.

If cdev_add() returns an error, the code calls
cdev_del() passing the STm->cdevs[rew] pointer as parameter;
the problem is that the pointer has not been initialized yet.

This patch fixes the problem by moving the STm->cdevs[rew] pointer
initialization before the call to cdev_add().
It also sets STm->devs[rew] and STm->cdevs[rew] to NULL in
case of failure.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/st.c

@@ -4083,6 +4083,7 @@ static int create_one_cdev(struct scsi_tape *tape, int mode, int rew)
 	}
 	cdev->owner = THIS_MODULE;
 	cdev->ops = &st_fops;
+	STm->cdevs[rew] = cdev;
 
 	error = cdev_add(cdev, cdev_devno, 1);
 	if (error) {
@@ -4091,7 +4092,6 @@ static int create_one_cdev(struct scsi_tape *tape, int mode, int rew)
 		pr_err("st%d: Device not attached.\n", dev_num);
 		goto out_free;
 	}
-	STm->cdevs[rew] = cdev;
 
 	i = mode << (4 - ST_NBR_MODE_BITS);
 	snprintf(name, 10, "%s%s%s", rew ? "n" : "",
@@ -4110,8 +4110,9 @@ static int create_one_cdev(struct scsi_tape *tape, int mode, int rew)
 	return 0;
 out_free:
 	cdev_del(STm->cdevs[rew]);
-	STm->cdevs[rew] = NULL;
 out:
+	STm->cdevs[rew] = NULL;
+	STm->devs[rew] = NULL;
 	return error;
 }