Fix Wii disc partitions

This is magumagu's [dvd-low-open-partition](https://github.com/magumagu/dolphin/tree/dvd-low-open-partition) rebased on top of a recent version of master, requested by JMC47. Fixes Masterpieces in Super Smash Bros. Brawl.
This should check that tmdSz is sane, rather than just blindly allocating up to 4GB of memory.

Huh, I didn't realize this branch never got merged.