Duke3D: Demo playback crashes after roughly 3 seconds

To reproduce:

1. Enable demo recording and start any map (doesn't matter which, also occurs on completely empty ones)
2. Play for roughly 5 seconds, then quit to title.
3. Demo playback will crash the game after roughly 3 seconds, and "out of sync" messages are visible shortly before that.

This started with SVN r7112 (commit 5132b41f). Seems to have been an unintended regression while cleaning up code, so nothing related to recent savegame changes.

Debug crashlog: eduke32.crash.log