MassProperties::from_compound produces NaN values if total mass is 0

[Demindiro]

Due to a division by 0 in Sum<MassProperties> it is possible to get NaN values.

parry/src/mass_properties/mass_properties.rs

Line 386 in 8241547

total_com /= total_mass;

This can happen when e.g. trying to compute the MassProperties of a HeighField.

I believe the sum function should either panic or return as soon as it detects the total mass is 0.

Reproduction project:

// src/main.rs

use rapier3d::pipeline::*;
use rapier3d::dynamics::*;
use rapier3d::geometry::*;
use rapier3d::na::*;

fn main() {

	let mut pipeline = PhysicsPipeline::new();
	let gravity = Vector3::new(0.0, -9.81, 0.0);
	let integration_parameters = IntegrationParameters::default();
	let mut broad_phase = BroadPhase::new();
	let mut narrow_phase = NarrowPhase::new();
	let mut bodies = RigidBodySet::new();
	let mut colliders = ColliderSet::new();
	let mut joints = JointSet::new();
	let physics_hooks = ();
	let event_handler = ();

	let body = RigidBodyBuilder::new_dynamic().translation(0.0, 1.01, 0.0).build();
	let handle = bodies.insert(body);
	let collider = ColliderBuilder::ball(1.0).build();
	colliders.insert(collider, handle, &mut bodies);

	let shape = SharedShape::heightfield(DMatrix::zeros(10, 10), Vector3::new(9.0, 1.0, 9.0));
	let collider = ColliderBuilder::new(shape.clone()).build();
	let mut body = RigidBodyBuilder::new_static().build();
	let position = *body.position();
	let mp = MassProperties::from_compound(1.0, &vec![(position, shape)][..]);
	dbg!(collider.mass_properties()); // This is fine
	dbg!(mp); // This has NaN center of mass
	body.set_mass_properties(mp, true);
	let handle = bodies.insert(body);
	colliders.insert(collider, handle, &mut bodies);

	loop {
		pipeline.step(
			&gravity,
			&integration_parameters,
			&mut broad_phase,
			&mut narrow_phase,
			&mut bodies,
			&mut colliders,
			&mut joints,
			&physics_hooks,
			&event_handler,
		);
		for (_, b) in bodies.iter() {
			if b.is_dynamic() {
				// The position of the ball also becomes NaN due to this
				println!("{}", b.position());
				assert!(!b.position().translation.x.is_nan());
			}
		}
	}
}

# Cargo.toml

[package]
name = "rapier3d_heightfield_nan"
version = "0.1.0"
authors = ["David Hoppenbrouwers <david@salt-inc.org>"]
edition = "2018"

[dependencies]
rapier3d = "*"