Allow SynthDrum to be run without LVL plugged in

[holgerschurig]

Code like this --- in SynthDrum::step() --- will allow this:

    // process sounds
    float lvl = inputs[IN_LEVEL].active ? clamp(inputs[IN_LEVEL].value / CV_MAX, 0.0, 1.0) : 1.0;
    outputs[OUTPUT_AUDIO].value = GetAudio() * AUDIO_MAX * lvl;

[mschack]

good idea