If __builtin:item's on_step() makes the dropped item invalid, e.g. by removing it, a later get_velocity() returns nil, which leads to a crash. After this change, the overridden on_step() aborts early if the object became invalid.
For a description of this Minetest mod, see https://forum.minetest.net/viewtopic.php?f=9&t=10271.