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: liujunjie <liujunjie23@huawei.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Message-id: 20180607080237.12360-1-liujunjie23@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>