net/vhost-net: do not assert on null pointer return from tap_get_vhost_net()

When 'vhost=off' or no vhost specific options at all are passed for the tap
net-device backend, tap_get_vhost_net() can return NULL. The function
net_init_tap_one() does not call vhost_net_init() on such cases and therefore
vhost_net pointer within the tap device state structure remains NULL. Hence,
assertion here on a NULL pointer return from tap_get_vhost_net() would not be
correct. Remove it and fix the crash generated by qemu upon initialization in
the following call chain :

qdev_realize() -> pci_qdev_realize() -> virtio_device_realize() ->
virtio_bus_device_plugged() -> virtio_net_get_features() -> get_vhost_net()

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: Cédric Le Goater <clg@redhat.com>
Fixes: 0e994668d00c9c ("vhost_net: add an assertion for TAP client backends")
Reported-by: Cédric Le Goater <clg@redhat.com>
Report: <abab7a71-216d-b103-fa47-70bdf9dc0080@redhat.com>
Signed-off-by: Ani Sinha <anisinha@redhat.com>
Message-Id: <20230628112804.36676-1-anisinha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/net/vhost_net.c

@@ -507,7 +507,12 @@ VHostNetState *get_vhost_net(NetClientState *nc)
     switch (nc->info->type) {
     case NET_CLIENT_DRIVER_TAP:
         vhost_net = tap_get_vhost_net(nc);
-        assert(vhost_net);
+        /*
+         * tap_get_vhost_net() can return NULL if a tap net-device backend is
+         * created with 'vhost=off' option, 'vhostforce=off' or no vhost or
+         * vhostforce or vhostfd options at all. Please see net_init_tap_one().
+         * Hence, we omit the assertion here.
+         */
         break;
 #ifdef CONFIG_VHOST_NET_USER
     case NET_CLIENT_DRIVER_VHOST_USER: