[SCSI] sym53c8xx: Fix NULL pointer dereference in slave_destroy

sym53c8xx_slave_destroy unconditionally assumes that sym53c8xx_slave_alloc has
succesesfully allocated a sym_lcb. This can lead to a NULL pointer dereference
(exposed by commit 4e6c82b3).
Signed-off-by: NStratos Psomadakis <psomas@gentoo.org>
Cc: stable@vger.kernel.org
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>

drivers/scsi/sym53c8xx_2/sym_glue.c

@@ -839,6 +839,10 @@ static void sym53c8xx_slave_destroy(struct scsi_device *sdev)
 	struct sym_lcb *lp = sym_lp(tp, sdev->lun);
 	unsigned long flags;
 
+	/* if slave_alloc returned before allocating a sym_lcb, return */
+	if (!lp)
+		return;
+
 	spin_lock_irqsave(np->s.host->host_lock, flags);
 
 	if (lp->busy_itlq || lp->busy_itl) {