Gen 1: Def and SpD under screens roll over 1024

Due how the game calculates the stats, if a Pokemon gets its
Defense or Special Defense over 1024 using Reflect or Light Screen,
the stat will roll over and count from 0.
This is a rare case, such as +def Snorlax under Reflect, but relevant nonetheless.

It would be better to change the engine of Gen 1 to work the same way it works on the Game Boy,
but this has the same effect and will do for now.
For future reference, this is the exact mechanics we must follow:
http://www.smogon.com/forums/threads/past-gens-research-thread.3506992/#post-5878612