MultyBody with FixedJoint causes a panic when only one part is attached to it

[Kritzefitz]

The following program panics with the message below:

let mut world = World::new();

let shape = ShapeHandle::new(Cuboid::new(Vector3::repeat(0.5)));
let collider = ColliderDesc::new(shape)
    .density(1.0);

let top_joint = FixedJoint::new(Isometry3::identity());
let mut body_desc = MultibodyDesc::new(top_joint)
    .collider(&collider)
    .parent_shift(Vector3::repeat(1.0));

/*let sub_joint = PrismaticJoint::new(Vector3::x_axis(), 0.0);
body_desc.add_child(sub_joint)
    .set_parent_shift(Vector3::repeat(1.0))
    .add_collider(&collider);*/

body_desc.build(&mut world);

Testbed::new(world).run()

thread 'main' panicked at 'Matrix slicing out of bounds.', /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nalgebra-0.18.0/src/base/matrix_slice.rs:227:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
             at /usr/src/rustc-1.34.2/src/libstd/panicking.rs:412
   6: nalgebra::base::matrix_slice::<impl nalgebra::base::matrix::Matrix<N, R, C, S>>::assert_slice_index
             at ./<::std::macros::panic macros>:3
   7: nalgebra::base::matrix_slice::<impl nalgebra::base::matrix::Matrix<N, R, C, S>>::columns_generic
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nalgebra-0.18.0/src/base/matrix_slice.rs:410
   8: nalgebra::base::matrix_slice::<impl nalgebra::base::matrix::Matrix<N, R, C, S>>::fixed_columns
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nalgebra-0.18.0/src/base/matrix_slice.rs:391
   9: nalgebra::base::matrix_slice::<impl nalgebra::base::matrix::Matrix<N, R, C, S>>::column
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nalgebra-0.18.0/src/base/matrix_slice.rs:360
  10: nalgebra::base::blas::<impl nalgebra::base::matrix::Matrix<N, D1, D1, S>>::quadform_with_workspace
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nalgebra-0.18.0/src/base/blas.rs:1487
  11: nalgebra::base::blas::<impl nalgebra::base::matrix::Matrix<N, D1, D1, S>>::quadform
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nalgebra-0.18.0/src/base/blas.rs:1532
  12: <nphysics3d::object::multibody::Multibody<N>>::update_inertias
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nphysics3d-0.11.1/src/object/multibody.rs:465
  13: <nphysics3d::object::multibody::Multibody<N>>::update_dynamics
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nphysics3d-0.11.1/src/object/multibody.rs:381
  14: <nphysics3d::object::multibody::Multibody<N> as nphysics3d::object::body::Body<N>>::update_dynamics
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nphysics3d-0.11.1/src/object/multibody.rs:841
  15: <nphysics3d::world::world::World<N>>::step
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nphysics3d-0.11.1/src/world/world.rs:238
  16: <nphysics_testbed3d::testbed::Testbed as kiss3d::window::state::State>::step
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nphysics_testbed3d-0.5.0/src/testbed.rs:552
  17: kiss3d::window::window::Window::do_render_with_state
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/kiss3d-0.19.0/src/window/window.rs:597
  18: kiss3d::window::window::Window::render_loop::{{closure}}
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/kiss3d-0.19.0/src/window/window.rs:581
  19: <kiss3d::window::gl_canvas::GLCanvas as kiss3d::window::canvas::AbstractCanvas>::render_loop
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/kiss3d-0.19.0/src/window/gl_canvas.rs:67
  20: kiss3d::window::canvas::Canvas::render_loop
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/kiss3d-0.19.0/src/window/canvas.rs:31
  21: kiss3d::window::window::Window::render_loop
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/kiss3d-0.19.0/src/window/window.rs:581
  22: nphysics_testbed3d::testbed::Testbed::run
             at /home/sven/.cargo/registry/src/github.com-1ecc6299db9ec823/nphysics_testbed3d-0.5.0/src/testbed.rs:243
  23: reproduce::main
             at src/reproduce.rs:27
  24: std::rt::lang_start::{{closure}}
             at /usr/src/rustc-1.34.2/src/libstd/rt.rs:64
  25: std::panicking::try::do_call
  26: __rust_maybe_catch_panic
  27: std::rt::lang_start_internal
  28: std::rt::lang_start
             at /usr/src/rustc-1.34.2/src/libstd/rt.rs:64
  29: main
  30: __libc_start_main
  31: _start

When I comment in the part that adds the sub part on a PrismaticJoint the panic goes away. I guess having a FixedJoint with only one part below it isn't really useful, as it could also be achieved by mounting a RigidBody to the ground, but I think at least the panic should give a proper message about the problem, as this is quite hard to debug.