Fix from upstream NBT package

Hello Fenixin,

long time no see :)

It looks like 1.7 changed something and lots of chunks get deleted, that are not corrupt. Using a more current nbt.py seems to fix it. I hope nothing is broken by using it. You may have a more detail look into that.

It's this version here: https://github.com/twoolie/NBT/commit/0f3ccffecbb16feca412094aa068ceef098eb273


Cheers,

Marcel