This commit fixs segment fault when rte_eth_dev_close() is called on
a virtio dev more than once. Assigning zero after free to avoids
freed memory to be accessed again.
Fixes: 69c80d4ef89b ("net/virtio: allocate queue at init stage")
Cc: stable@dpdk.org
Signed-off-by: Huanle Han <hanxueluo@gmail.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>