Crash when starting a map with several scrollers

I'm on a Mac (if relevant) and trying to load a test map with several types of scrollers. I'm using the DRDTeam devbuild of gzdoom-g4.8pre-83-g54ad3433b.zip. The level contains several uses of the Scroll_Texture_Offsets action special, with both documented and undocumented flags being used (the undocumented ones are necessary to support the equivalent MBF21 Boom specials).

The attached zip contains the map and the crash log.

scroll-udmf.zip

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The problem is in this line. The condition should be to check if the value of it.Next() is non-negative, otherwise it eventually returns -1 and everything goes boom because this value is used as an array index. I've submitted a PR to fix this here.