net/virtio: fix crash while freeing mbufs

virtio_dev_free_mbufs was recently modified to free the
virtqueues but failed to check whether the array was
allocated.  Added a check to ensure vqs was non-null.

Fixes: bdb32afbb610 ("net/virtio: rationalize queue flushing")

Signed-off-by: David Harton <dharton@cisco.com>
Reviewed-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

drivers/net/virtio/virtio_ethdev.c

@@ -1975,6 +1975,9 @@ static void virtio_dev_free_mbufs(struct rte_eth_dev *dev)
 	struct rte_mbuf *buf;
 	int queue_type;
 
+	if (hw->vqs == NULL)
+		return;
+
 	for (i = 0; i < nr_vq; i++) {
 		vq = hw->vqs[i];
 		if (!vq)