Code authored by Michael B Johnson (drwave). Reviewed by Sybren and makowalski. This patch addresses a crash that occurs when exporting a scene to a USD file, when that scene includes a point cloud object or hair. Added OB_POINTCLOUD and OB_CURVES enums and a default case statement in the switch statement in USDHierarchyIterator::create_data_writer, to avoid dereferencing a NULL pointer. Differential Revision: https://developer.blender.org/D16776