wifi/iwd: check priv->dbus_obj in is_available

is_available would recently return true after IWD had disconnected
if a connection was active because it would check that
priv->dbus_station_proxy was non-NULL (i.e. that the DBus interface was
still visible, which it wasn't) but that check would be overridden
if the NMDevice state was activated.  Now require priv->dbus_obj to be
non-NULL, which would even be enough on its own although I'm leaving the
previous check there too to catch potential IWD states we don't support
in which priv->dbus_station_proxy is NULL without an active connection.

src/devices/wifi/nm-device-iwd.c

@@ -952,7 +952,8 @@ is_available (NMDevice *device, NMDeviceCheckDevAvailableFlags flags)
 	 * We call nm_device_queue_recheck_available whenever
 	 * priv->enabled changes or priv->dbus_station_proxy changes.
 	 */
-	return    priv->enabled
+	return    priv->dbus_obj
+	       && priv->enabled
 	       && (   priv->dbus_station_proxy
 	           || (state >= NM_DEVICE_STATE_CONFIG && state <= NM_DEVICE_STATE_DEACTIVATING));
 }