net/virtio: fix null pointer dereference

net/virtio: fix null pointer dereference

There is a logic bug in this code, that could lead to null pointer
dereference when cvq is NULL. Fix this problem by changing logic
&& to logic ||.

   >> CID 127480:  Null pointer dereferences  (FORWARD_NULL)
   >> Dereferencing null pointer "cvq".
   	if (!cvq && !cvq->vq) {
            ...
        }

Coverity issue: 127480
Fixes: 01ad44fd374f ("net/virtio: split Rx/Tx queue")
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>