The rxq/txq for the queue_release callback could be NULL, say when
rte_eth_dev_configure() fails that the queue is not setup at all.
Do a simple NULL check would fix the crash issue.
Fixes: 01ad44fd374f ("net/virtio: split Rx/Tx queue")
Reported-by: Olivier Matz <olivier.matz@6wind.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>