For some specific out of bounds values, the volume calculation in
minetest.find_nodes_in_area() is off by about four million nodes.
Unfortunately that behaviour made it trivial to crash Mineclonia,
as Minetest immediately crashes upon encountering large numbers.
This commit introduces a wrapper around minetest.find_nodes_in_area()
which should avoid a crash. Additionally, three self tests are executed
when a server starts; they crash Mineclonia in case the workaround fails.
