Tweak again the null move pruning preconditions.
STC:
LLR: 2.96 (-2.94,2.94) [0.50,4.50]
Total: 19675 W: 4430 L: 4169 D: 11076
http://tests.stockfishchess.org/tests/view/
5d52bc0e0ebc5925cf108300
LTC:
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 73895 W: 12496 L: 12114 D: 49285
http://tests.stockfishchess.org/tests/view/
5d52dcbc0ebc5925cf108552
Closes https://github.com/official-stockfish/Stockfish/pull/2268
Bench:
3690065