Fix: avoid error while emitting 'EndShapeContact' events.

[dtysky]

Hi.

I used events 'beginContact', 'endContact', 'beginShapeContact' and 'endShapeContact' and found that if I remove body or shapes in the callback of 'beginContact' like this:

world.addEventListener('beginContact', args => {
      world.remove(args.bodyB);
});

Then an error will be thrown:

cannon.js:14503 Uncaught TypeError: Cannot read property 'body' of undefined
at World.emitContactEvents (cannon.js:14503)
at World.internalStep (cannon.js:14312)
at World.step (cannon.js:14031)

So I add this condition to avoid it.