Use increased LMR horizont only after a null move

Revert to LMR horizont of 2 plies. Only if parent move
is a null move increase to 3 so to avoid the bad combination
of null move reduction + LMR reduction. This is a more
aggressive patch then previous one, but it seems we are
going in the wromg direction.

After 531 games result is not good:

Mod vs Orig  +123/=265/-143 48.12%  255.5/531  -13 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>