display: virtio-gpu-3d: check virgl capabilities max_size

Virtio GPU device while processing 'VIRTIO_GPU_CMD_GET_CAPSET'
command, retrieves the maximum capabilities size to fill in the
response object. It continues to fill in capabilities even if
retrieved 'max_size' is zero(0), thus resulting in OOB access.
Add check to avoid it.

Reported-by: Zhenhao Hong <zhenhaohong@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-id: 20161214070156.23368-1-ppandit@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
