Fix hang on missing state update handler

[sapir]

If a state's "update" stage is None, then the state run loop is infinite, so this fixes it.

[cart]

Ooh very nice catch. I think we can make this a bit simpler by just adding a break in the event that we don't have a "next" and "update" isn't set:

            if let Some(next_stage) = next_stage {
                if next_stage != current_stage {
                    if let Some(exit_current) = self
                        .stages
                        .get_mut(&current_stage)
                        .and_then(|stage| stage.exit.as_mut())
                    {
                        exit_current.run(world, resources);
                    }
                }

                if let Some(enter_next) = self
                    .stages
                    .get_mut(&next_stage)
                    .and_then(|stage| stage.enter.as_mut())
                {
                    enter_next.run(world, resources);
                }
            } else if let Some(update_current) = self
                .stages
                .get_mut(&current_stage)
                .and_then(|stage| stage.update.as_mut())
            {
                update_current.run(world, resources);
                break;
            } else {
                break;
            }

[sapir]

I thought this way it would show the flow more clearly: First it loops until it finds the current stage, then it calls "update"

edit: The commit before my force-push (e05bd97) looked more like your version

[cart]

Thats a fair point. At a glance you can tell that "update" only runs once. I'm sold!