ps2: check PS2Queue wptr pointer in post_load routine

In commit 802cbcb7, most issues have been fixed when qemu guest
migration. But the queue size still need to check whether is equal to
PS2_QUEUE_SIZE. If yes, the wptr should set as 0. Or, wptr would larger
than PS2_QUEUE_SIZE and never come back when ps2_queue_noirq is called.
This could lead to OOB access, add check to avoid it.
Signed-off-by: Nliujunjie <liujunjie23@huawei.com>
Reviewed-by: NGonglei <arei.gonglei@huawei.com>
Message-id: 20180607080237.12360-1-liujunjie23@huawei.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/input/ps2.c

@@ -927,7 +927,7 @@ static void ps2_common_post_load(PS2State *s)
 
     /* reset rptr/wptr/count */
     q->rptr = 0;
-    q->wptr = size;
+    q->wptr = (size == PS2_QUEUE_SIZE) ? 0 : size;
     q->count = size;
     s->update_irq(s->update_arg, q->count != 0);
 }