Structure blocks will not load structures below Y=0 or above Y=256

When attempting to load a structure with a structure block at any y level below 0; the structure will load with the lowest part at y=0 as though 0 was still the lower build limit.

Steps to Reproduce:

1. Create new world
2. Cheats turned on, Caves and cliffs turned on
3. /setblock 0 0 0 structure block

Observed Results:
Inside the block menu any Y offset less than zero is reset to zero.

Expected Results:
Y offset should go as low as -64 from that level.