To reproduce:
1. Open attached project
2. Open scene "CastleCoaster"
3. Build lighting

One of the threads will slowly (in 10-15 min.) eat up the available RAM until there is almost none left, then RAM usage will stay at 94-99% for 10-15 minutes, a few errors will be thrown (Failed executing external process for 'Clustering' job.) after which Unity will crash, with some other results, such as:
-Resetting the desktop wallpaper
-Changing the resolution to the smallest possible
-Crashing the graphics card
-Crashing other applications

Expected result: Unity doesn't crash, or at the very least doesn't produce the mentioned effects and displays a message explaining that it crashed because it ran out of RAM.