Fixed crash when deleting actor while holding backspace

[tritonas00]

Fixes #2860

It happened only on EV_COMMON_REMOVE_CURRENT_TRUCK, removing from top menu bar works fine.

[ohlidalp]

I think I know why this helps...
The 'backspace mode' does check if the player actor was removed:

rigs-of-rods/source/main/gameplay/RecoveryMode.cpp

Lines 39 to 44 in 6897c1f

	if (!App::GetGameContext()->GetPlayerActor())
	{
	m_advanced_vehicle_repair = false;
	m_advanced_vehicle_repair_timer = 0.0f;
	return;
	}

But if that code runs while DELETE message is already queued, then it's bust - it will queue the MODIFY message with a pointer to vehicle which will already be DELETE-d at the time of processing.
Well, I'm going to approve this for now, but it must eventually be redone like this:

typedef std::shared_ptr<Actor> ActorPtr;

enum ActorState { DELETED, /*existing ones*/ }

ActorManager:
    std::vector<ActorPtr> m_actors;

on MODIFY:
    // check if not already DELETED

on DELETE:
    // remove from ActorManager and set state to DELETED

That would also solve a potential issue with scripting - those may hold pointers to actors as well.
Unfortunately AngelScript doesn't support binding shared_ptr<>, so function proxies which check DELETED and nullptr would have to be added.