🐛 Incorrect (too negative) mate scores when being checkmated

[eduherminio]

Root cause of #1266.

Changes

• Avoid fail-high pruning while being checkmated
• If we're having no best moves, at least don't mess up and start from scratch the aspiration windows search to avoid chaining them

2 threads vs 2 threads

Score of Lynx-bugfix-start-from-scract-when-nomove-4827-win-x64 vs Lynx 4818 - main: 679 - 670 - 1431  [0.502] 2780
...      Lynx-bugfix-start-from-scract-when-nomove-4827-win-x64 playing White: 546 - 128 - 716  [0.650] 1390
...      Lynx-bugfix-start-from-scract-when-nomove-4827-win-x64 playing Black: 133 - 542 - 715  [0.353] 1390
...      White vs Black: 1088 - 261 - 1431  [0.649] 2780
Elo difference: 1.1 +/- 9.0, LOS: 59.7 %, DrawRatio: 51.5 %
SPRT: llr 0.403 (14.0%), lbound -2.25, ubound 2.89

Regular SPRT

Score of Lynx-bugfix-start-from-scract-when-nomove-4827-win-x64 vs Lynx 4818 - main: 2380 - 2409 - 4591  [0.498] 9380
...      Lynx-bugfix-start-from-scract-when-nomove-4827-win-x64 playing White: 1843 - 538 - 2309  [0.639] 4690
...      Lynx-bugfix-start-from-scract-when-nomove-4827-win-x64 playing Black: 537 - 1871 - 2282  [0.358] 4690
...      White vs Black: 3714 - 1075 - 4591  [0.641] 9380
Elo difference: -1.1 +/- 5.0, LOS: 33.8 %, DrawRatio: 48.9 %
SPRT: llr -0.0451 (-1.6%), lbound -2.25, ubound 2.89