Cycles: Fix regression introduced in c96a4c8

A few places still needed to be updated to use the new Mesh::num_triangles()
method; wrong number from triangles.size() was causing crashes.

intern/cycles/render/mesh.cpp

@@ -1485,7 +1485,7 @@ bool Mesh::need_attribute(Scene * /*scene*/, ustring name)
 
 void Mesh::tessellate(DiagSplit *split)
 {
-	int num_faces = triangles.size();
+	int num_faces = num_triangles();
 
 	add_face_normals();
 	add_vertex_normals();