Merge branch 'polyobj-setup-fixes' into 'master'

Polyobj setup fixes

If you're wondering why ERZ2 crashes lately, yes, it's my fault once again it turns out! Ideally we shouldn't have loose spawn points or anchors without an actual PolyObject to go with them in the first place, but this fix re-adds the safety check that prevented them from crashing the game before.

If it wasn't clear already, this fix is rather important, so please get in asap.

See merge request !7