Fix segfault when quitting a level while grabbing

[seanyeh]

Fixes #1739

Reproduction steps:
Jump on Mr ice block, grab, and abort level while grabbing. Segfaults

As explained in the issue, the player is destroyed before the grab listener. I believe it segfaults here:

supertux/src/supertux/game_object.cpp

Lines 54 to 56 in 1df160f

	for (const auto& entry : m_remove_listeners) {
	entry->object_removed(this);
	}

when the grab listener (associated with the now-destroyed player) does something.

By ungrabbing in the player destructor, we also remove this listener and avoid this issue. Let me know if this seems correct, thanks

[Semphriss]

I'll check this ASAP - the thing I'm afraid about is if the lamp gets destroyed first, then the player might cause a segfault now when ungrabbing the now-destroyed lamp.

[tobbi]

@Semphriss Did this work? From a quick glance, it looks like it should.

[Grumbel]

To avoid these issues in the future I would recommend not using pointers directly, but holding GameObject by their UID and using GameObjectManager::get_object_by_uid() to retrieve the pointer when needed. Makes it much easier to deal with disappearing objects.

[Semphriss]

On quick first try, it seems to have fixed the problem 👍