net/virtio: fix queues pointer check

net/virtio: fix queues pointer check

It is necessary to add pointer checking because in some case the
code will cause crash. For example, the code goes here before
memory allocation of rxvq is finished.

Fixes: 7365504f77e3 ("net/virtio: support guest announce")
Cc: stable@dpdk.org
Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>