etesian: Return NULL for out-of-bounds slice

This can occur when the placement area is not divisible by the slice height. There is a small slither at the top whose 'islice' is one outside of the _slices array. Return NULL as if y was outside the placement area.

Would appreciate some feedback if this is the correct approach or if there's a better solution here, like rejecting placement area heights not divisible by the slice height.