Compare mesh loop count with number of loop normals before reading the
loop normals.
Houdini doesn't always write the correct loop normals to Alembic. When a
mesh is animated and then replaced by a fluid simulation, Houdini will
still write the original mesh's loop normals, but the mesh
verts/loops/polys are from the simulation. In such cases the normals
cannot be mapped to the mesh, so it's better to ignore them.
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