Map generation limit: Fix checks for block/sector over-limit

Map generation limit: Fix checks for block/sector over-limit

Fix the maths that check if any part of a mapblock or sector is over the
set map_generation_limit.
Therefore avoid the loading of any over-limit blocks that were previously
generated when map_generation_limit was larger. The set limit can vary
for a world because it is not yet a per-world mapgen parameter, even when
it is sometimes it will be changed deliberately.
Therefore avoid a player being returned to world centre if they re-enter
a world while being over-limit.

Fix the createSector() crash caused by a mob spawning over-limit in an
over-limit mapblock