Fixed memsetting non-trivially-copyable types

Types stl_stats, stl_normal and SurfaceFillParams should not be zeroed by memset
This is not correct and also triggered warnings on gcc

src/libslic3r/Format/PRUS.cpp

@@ -246,7 +246,7 @@ static void extract_model_from_archive(
                 sscanf(normal_buf[2], "%f", &facet.normal(2)) != 1) {
                 // Normal was mangled. Maybe denormals or "not a number" were stored?
                 // Just reset the normal and silently ignore it.
-                memset(&facet.normal, 0, sizeof(facet.normal));
+                facet.normal = stl_normal::Zero();
             }
             facets.emplace_back(facet);
         }