Bugfix: random but frequent crashes after recent perimeter code refactoring

lib/Slic3r/Layer/PerimeterGenerator.pm

@@ -259,7 +259,8 @@ sub process {
                 || ($self->layer_id == 0 && $self->print_config->brim_width > 0);
         
         # append perimeters for this slice as a collection
-        $self->loops->append(Slic3r::ExtrusionPath::Collection->new(@loops));
+        $self->loops->append(Slic3r::ExtrusionPath::Collection->new(@loops))
+            if @loops;
     }
 }
 

lib/Slic3r/Print/GCode.pm

@@ -425,6 +425,8 @@ sub process_layer {
             {
                 my $extruder_id = $region->config->perimeter_extruder-1;
                 foreach my $perimeter_coll (@{$layerm->perimeters}) {
+                    next if $perimeter_coll->empty;  # this shouldn't happen but first_point() would fail
+                    
                     # init by_extruder item only if we actually use the extruder
                     $by_extruder{$extruder_id} //= [];
                     
@@ -447,6 +449,8 @@ sub process_layer {
             # throughout the code). We can redefine the order of such Collections but we have to 
             # do each one completely at once.
             foreach my $fill (@{$layerm->fills}) {
+                next if $fill->empty;  # this shouldn't happen but first_point() would fail
+                
                 # init by_extruder item only if we actually use the extruder
                 my $extruder_id = $fill->[0]->is_solid_infill
                     ? $region->config->solid_infill_extruder-1

xs/xsp/ExtrusionEntityCollection.xsp

@@ -24,6 +24,8 @@
     Clone<Point> last_point();
     int count()
         %code{% RETVAL = THIS->entities.size(); %};
+    bool empty()
+        %code{% RETVAL = THIS->entities.empty(); %};
     std::vector<size_t> orig_indices()
         %code{% RETVAL = THIS->orig_indices; %};
 %{