Fix ThinkerIterator Shenanigans

[XaserAcheron]

From this thread:

The damnum thinker destroys stuff when it receives an event. And unfortunately due to the setup this is precisely the next thinker to be iterated by the ThinkerIterator which as a result loses its link to the chain.

I cannot fix this, but the workaround should be that SpawnAndDestroy does not actually destroy the thinker but only flags it for destruction in its next Tick call.