[Fixed] BlockThingsIterator gives between sector portal actors twice

[Gutawer]

I've noticed that a ZScript BlockThingsIterator gives actors that are in between a sector portal boundary twice, meaning that any code using a BlockThingsIterator would run code on these actors twice, when they were only meant to run once. I've attached a pk3 exemplifying this (you'll have to move around a bit to become properly between portal boundaries, then the Player and Cacodemon will appear twice in the printed BlockThingsIterator output - if you crouch, each actor will only be printed once, which is what is supposed to happen).

[IvanDobrovski]

I've recently encountered issue in the form of a very large monster occupying parts in both sector portals and being hit with a substantially large radius explosion damage, taking the damage twice. Hopefully this gets looked at.

[Graf Zahl]

fixed