🐛 Don't prune moves in regular search while being checkmated

[eduherminio]

Still didn't solve the illegal moves issue, but..

Test  | bugfix/no-pruning-negative-checkmate-score
Elo   | 2.42 +- 4.03 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.90 (-2.25, 2.89) [-5.00, 0.00]
Games | 10928: +2867 -2791 =5270
Penta | [208, 1238, 2477, 1352, 189]
https://openbench.lynx-chess.com/test/779/