Use slightly different formula for stat score when position is in check
Use formula that is closer to movepicker one.
Passed STC:
https://tests.stockfishchess.org/tests/view/67cffb337be98c1ad9b021ee
LLR: 2.99 (-2.94,2.94) <0.00,2.00>
Total: 250432 W: 64978 L: 64343 D: 121111
Ptnml(0-2): 795, 29390, 64159, 30129, 743
Passed LTC:
https://tests.stockfishchess.org/tests/view/67d3905d517865b4a2dfce8a
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 140004 W: 35742 L: 35215 D: 69047
Ptnml(0-2): 60, 15111, 39151, 15602, 78
closes https://github.com/official-stockfish/Stockfish/pull/5930
Bench: 2147336