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

…the tallest.

#4309
#4313

src/libslic3r/GCode.cpp

@@ -3058,8 +3058,10 @@ 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) {
@@ -3070,6 +3072,7 @@ 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())
@@ -3082,7 +3085,7 @@ LayerResult GCode::process_layer(
  m_object_sequentially_printed.insert(object_layer->object());
  print.set_status(int((layer.id() * 100) / nb_layers),
  std::string(L("Generating G-code layer %s / %s for object %s / %s")),
- std::vector<std::string>{std::to_string(layer.id()), std::to_string(nb_layers), m_object_sequentially_printed.size(), print.num_object_instances()},
+ std::vector<std::string>{std::to_string(layer.id()), std::to_string(nb_layers), std::to_string(m_object_sequentially_printed.size()), std::to_string(print.num_object_instances())},
  PrintBase::SlicingStatus::DEFAULT | PrintBase::SlicingStatus::SECONDARY_STATE);
  } else {
  print.set_status(int((layer.id() * 100) / layer_count()),