vhost: check for vhost_ops before using.

'vhost_set_vring_enable()' tries to call function using pointer to
'vhost_ops' which can be already zeroized in 'vhost_dev_cleanup()'
while vhost disconnection.

Fix that by checking 'vhost_ops' before using. This fixes QEMU crash
on calling 'ethtool -L eth0 combined 2' if vhost disconnected.

Signed-off-by: Ilya Maximets <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>

hw/net/vhost_net.c

@@ -428,7 +428,7 @@ int vhost_set_vring_enable(NetClientState *nc, int enable)
 
     nc->vring_enable = enable;
 
-    if (vhost_ops->vhost_set_vring_enable) {
+    if (vhost_ops && vhost_ops->vhost_set_vring_enable) {
         return vhost_ops->vhost_set_vring_enable(&net->dev, enable);
     }