fixed 3 count em 3 undefined behaviour bugs

#1 - copy_n for reading the file doesn't care if the iterator hits the
end of the file and continues to increment and dereference it

#2 In loadSprite, even with x%64, x+i can still go out of bounds (hence
(x+i)%64)

#3 When copying I to V0-Vx and vice-versa, V[I+i] was being logged
instead of V[i]. Cue an I value of 754. Oops. Way out of bounds.