Fix under-promotion hash mixup

Fix under-promotion hash mixup

The hash table was invalidated on under-promotion by adding the promotion
'deficit' to the J hash key. But for some fairy-pieces (in particular
Spartan G vs W) this difference apparently caused false hits in the
null-move search, leading to misqualification of positions as stalemate.
Presumably because the J key is also modified by stm, and different
stm and promo-deficit could cancel each other. In stead we now modify
the Z hash key with the promo-deficit.