Tweak return value in futility pruning
In futility pruning, return the average between eval and beta.
Passed STC:
https://tests.stockfishchess.org/tests/view/65680bb6136acbc5735521d7
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 15200 W: 3926 L: 3642 D: 7632
Ptnml(0-2): 36, 1699, 3867, 1941, 57
Passed LTC:
https://tests.stockfishchess.org/tests/view/656817fc136acbc573552304
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 200376 W: 49700 L: 49036 D: 101640
Ptnml(0-2): 110, 22584, 54137, 23246, 111
closes https://github.com/official-stockfish/Stockfish/pull/4897
Bench: 1403703