Prevent unknown variant getting button in -ncp mode

Prevent unknown variant getting button in -ncp mode

Normally we fake the engines play the requested variant, for the benefit
of engines that do not send a variants feature (e.g. v1 engines). But this
should not be done if there is no engine, as it would lead XBoard to
believe an unknown variant name is an engine-defined variant supported
by a currently loaded engine, and create a button for it in the New Variant
dialog.