Guard tt depth edits from racy underflows
Two recent patches, involving editing TTEs, introduced potential racy underflows. Guarding against these doesn't appear to gain elo but does improve sanity.
fixes https://github.com/official-stockfish/Stockfish/issues/6847
STC: https://tests.stockfishchess.org/tests/view/6a1c8c4b818cacc1db0ad158
LLR: 3.85 (-2.94,2.94) <-1.75,0.25>
Total: 125984 W: 32248 L: 32055 D: 61681
Ptnml(0-2): 292, 13678, 34866, 13857, 299
SMP STC: https://tests.stockfishchess.org/tests/view/6a1fb2ba818cacc1db0ad2e2
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 104272 W: 26524 L: 26385 D: 51363
Ptnml(0-2): 103, 11650, 28494, 11783, 106
closes https://github.com/official-stockfish/Stockfish/pull/6883
No functional change