Wrong mate scores correction

[MJZ1977]

This is a try correct some wrong mate scores observed on fishtest when SF exits at depth 1, generally when there is only one legal move.

See Issue #2707

STC non regression
LLR: 2.95 (-2.94,2.94) {-1.50,0.50}
Total: 32672 W: 6304 L: 6170 D: 20198
Ptnml(0-2): 498, 3688, 7854, 3774, 522
https://tests.stockfishchess.org/tests/view/5ed4cd6ef29b40b0fc95a76b

Bench: 4704615

[vondele]

Thanks, I went with the 1ms variant as discussed in the issue.