Panic when adding multi body joint dynamically.

[newDINO]

Panics when adding multi body joint after initialization between steps.
Backtrace info:

thread 'main' (542881) panicked at crates/rapier3d/../../src/dynamics/joint/multibody_joint/multibody.rs:223:32:
attempt to subtract with overflow
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic_const::panic_const_sub_overflow
   3: rapier3d::dynamics::joint::multibody_joint::multibody::Multibody::append
   4: rapier3d::dynamics::joint::multibody_joint::multibody_joint_set::MultibodyJointSet::insert
   5: all_examples3::debug_add_multibody_joint3::init_world::{{closure}}
   6: rapier_testbed3d::harness::Harness::step_with_graphics
   7: rapier_testbed3d::testbed::update_testbed
   8: <bevy_ecs::system::function_system::FunctionSystem<Marker,F> as bevy_ecs::system::system::System>::run_unsafe

I created an example for reproduction at newDINO/rapier@6da6777. Run cargo run --bin all_examples3 and select "(Debug) add multibody joint".
As is shown in the example, adding joint during initialization works fine.

Changing multi body joints to impulse joints also shows no problem.