[bugfix] Guard is_decisive() to avoid assert triggers
This PR fixes an oversight in #6818. At least in multithreaded searches the TT scores can be corrupted, and accessing them is racy.
Passed non-reg STC:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 61792 W: 15609 L: 15428 D: 30755
Ptnml(0-2): 134, 6534, 17373, 6727, 128
https://tests.stockfishchess.org/tests/view/6a0b15396524d21ee79b868b
We also increase the number of the threads in our instrumented CI tests to make these assert failures for buggy patches more likely in future.
closes https://github.com/official-stockfish/Stockfish/pull/6830
No functional change