handle stalemate correctly

[nli33]

Give stalemate an accurate score of 0 (instead of -Infinity) in alphaBetaInner

Avoids taking stalemates when we are winning since a stalemate is no longer viewed as a win for us

Results of Silverfish-dev vs Silverfish-nnue (10+0.1, NULL, NULL, 8moves_v3.pgn):
Elo: 6.40 +/- 4.25, nElo: 11.56 +/- 7.67
LOS: 99.84 %, DrawRatio: 53.56 %, PairsRatio: 1.14
Games: 7876, Wins: 1470, Losses: 1325, Draws: 5081, Points: 4010.5 (50.92 %)
Ptnml(0-2): [85, 770, 2109, 863, 111], WL/DD Ratio: 0.22
LLR: 2.95 (100.3%) (-2.94, 2.94) [0.00, 5.00]
--------------------------------------------------
SPRT ([0.00, 5.00]) completed - H1 was accepted

Player: Silverfish-dev
  Timeouts: 1
  Crashed: 0
Player: Silverfish-nnue
  Timeouts: 5
  Crashed: 0

(todo: detect threefolds)