Fix freeze/crash happening when slicing with 0 perimeters

src/libslic3r/LayerRegion.cpp

@@ -355,6 +355,8 @@ void LayerRegion::process_external_surfaces(const Layer *lower_layer, const Poly
         shell_width += 0.5f * external_perimeter_flow.scaled_width() + external_perimeter_flow.scaled_spacing();
         shell_width += perimeter_flow.scaled_spacing() * (num_perimeters - 1);
     } else {
+        // TODO: Maybe there is better solution when printing with zero perimeters, but this works reasonably well, given the situation
+        shell_width = SCALED_EPSILON;
     }
 
     // Scaled expansions of the respective external surfaces.