make some assertions to hopefully ensure sanity here

..because i sure don't have any.

seems i'm getting a use after free deep inside here, but it doesn't
make sense to me because i see it remove the entity(ies), and then
that should be all for that tick, next tick it should do processing..
and it should've already had it's active list of entities (inside the
system base class) taken care of. somehow it's still wanting to process
on that same damn entity..idk wtf is going on with that.