Fix ponder hang

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 87%. Comparing base (caa7021) to head (921171f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #214 +/- ## ===================================== + Coverage 87% 87% +1% ===================================== Files 32 32 Lines 4283 4274 -9 Branches 4283 4274 -9 ===================================== - Hits 3684 3678 -6 + Misses 583 579 -4 - Partials 16 17 +1 ```

[github-actions[bot]]

Against master [hash=64; threads=1]: 🆗 Elo difference: -2.90 +/- 22.15, LOS: 39.89 %, DrawRatio: 36.50 %

[github-actions[bot]]

Against v1.4.0 [hash=256; threads=4]: ✅ Elo difference: 47.19 +/- 40.48, LOS: 98.90 %, DrawRatio: 30.50 %

[github-actions[bot]]

Against master [hash=256; threads=4]: 🆗 Elo difference: -24.36 +/- 37.74, LOS: 10.25 %, DrawRatio: 39.00 %