Fix crash when support bottom interface layer is set to same as top (#8955)

* Fix crash when support bottom interface layer option is set to `same as top` (#8604)

Author: Noisyfox

src/libslic3r/Support/TreeSupport.cpp

@@ -1948,7 +1948,7 @@ void TreeSupport::draw_circles()
     // generate areas
     const coordf_t layer_height = config.layer_height.value;
     const size_t   top_interface_layers = config.support_interface_top_layers.value;
-    const size_t   bottom_interface_layers = config.support_interface_bottom_layers.value;
+    const size_t   bottom_interface_layers = config.support_interface_bottom_layers.value < 0 ? top_interface_layers : config.support_interface_bottom_layers.value;
     const double nozzle_diameter = m_object->print()->config().nozzle_diameter.get_at(0);
     const coordf_t line_width = config.get_abs_value("support_line_width", nozzle_diameter);
     const coordf_t line_width_scaled           = scale_(line_width);
@@ -2428,8 +2428,8 @@ void TreeSupport::drop_nodes()
     const size_t tip_layers = base_radius / layer_height; //The number of layers to be shrinking the circle to create a tip. This produces a 45 degree angle.
     const coordf_t radius_sample_resolution = m_ts_data->m_radius_sample_resolution;
     const bool support_on_buildplate_only = config.support_on_build_plate_only.value;
-    const size_t bottom_interface_layers = config.support_interface_bottom_layers.value;
     const size_t top_interface_layers = config.support_interface_top_layers.value;
+    const size_t bottom_interface_layers = config.support_interface_bottom_layers.value < 0 ? top_interface_layers : config.support_interface_bottom_layers.value;
     SupportNode::diameter_angle_scale_factor = diameter_angle_scale_factor;
     float        DO_NOT_MOVER_UNDER_MM       = is_slim ? 0 : 5;                     // do not move contact points under 5mm