Fix crash when multiple objects are printed, and the first one isn't …

…the tallest.

#4309
#4313

src/libslic3r/GCode.cpp

@@ -3019,8 +3019,10 @@ LayerResult GCode::process_layer(
  const Layer *object_layer = nullptr;
  const SupportLayer *support_layer = nullptr;
  const SupportLayer *raft_layer = nullptr;
- const size_t layer_id = layers.front().layer()->id();
+ /*const*/ size_t layer_id = size_t(-1);
  for (const LayerToPrint &l : layers) {
+ if(l.layer())
+ layer_id = l.layer()->id();
  if (l.object_layer && ! object_layer)
  object_layer = l.object_layer;
  if (l.support_layer) {
@@ -3031,6 +3033,7 @@ LayerResult GCode::process_layer(
  }
  assert(l.layer() == nullptr || layer_id == l.layer()->id());
  }
+ assert(layer_id < layer_count());
  const Layer &layer = (object_layer != nullptr) ? *object_layer : *support_layer;
  LayerResult result { {}, layer.id(), false, last_layer, false};
  if (layer_tools.extruders.empty())