normal estimation on boundary fix

src/polymesh/algorithms/normal_estimation.hh

@@ -19,7 +19,13 @@ template <class Vec3, class IsHardEdgeF>
 
     auto const hard_edges = m.edges().map([&](edge_handle e) { return e.is_boundary() ? true : is_hard_edge(e); });
 
-    return m.halfedges().map([&](halfedge_handle h) {
+    return m.halfedges().map(
+        [&](halfedge_handle h)
+        {
+            if (h.is_boundary())
+                return Vec3{};
+
+            POLYMESH_ASSERT(h.face().is_valid());
             Vec3 n = face_normals[h.face()];
 
             auto h0 = h;