Reduce null move pruning one ply less in odd threads
Threads with an odd index use a null move reduction that is one ply smaller, so half of the threads verify null move cutoffs more carefully.
Passed SMP STC:
https://tests.stockfishchess.org/tests/view/6ac8bd225349156614d940fb
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 279256 W: 71709 L: 71076 D: 136471
Ptnml(0-2): 284, 30895, 76665, 31472, 312
Passed SMP LTC:
https://tests.stockfishchess.org/tests/view/6ad093f0733e627f7058c29d
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 328158 W: 85355 L: 84497 D: 158306
Ptnml(0-2): 78, 31716, 99656, 32528, 101
No functional change for a single thread.
closes https://github.com/official-stockfish/Stockfish/pull/7264
No functional change