update_old_parent Doesn't check to see if parent actually exists.

[StarArawn]

Bevy version

0.10.1

What you did

I have a setup like this:

- parent 1
  - child 1

What went wrong

I have a change like this:

1. Spawn parent 2
2. despawn parent 1
3. add child 1 as a child of parent 2

And I get this error:
thread 'main' panicked at 'Entity 43v0 does not exist'

Additional information

It looks like we just need to check if the parent exists.

[StarArawn]

Here is a reproducible example:

use bevy::prelude::*;

fn main() {
    App::new()
    .add_plugins(DefaultPlugins)
    .add_startup_system(bug)
    .run();
}

fn bug(world: &mut World) {
    let parent_0 = world.spawn_empty().id();
    let child_0 = world.spawn_empty().id();

    world.entity_mut(parent_0).add_child(child_0);

    world.entity_mut(parent_0).despawn();

    let parent_1 = world.spawn_empty().id();
    world.entity_mut(parent_1).add_child(child_0);
}

[alice-i-cecile]

I agree that this is a bug: this shouldn't crash.