Avoid changing bestvalue
in the case the ttValue contains mate scores, do not return them as bestValue, since they are not proven.
passed STC
https://tests.stockfishchess.org/tests/view/665b1ea5586058766677cfa3
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 58912 W: 15319 L: 15130 D: 28463
Ptnml(0-2): 141, 6562, 15854, 6765, 134
passed LTC:
https://tests.stockfishchess.org/tests/view/665b2712586058766677cfc4
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 141666 W: 35976 L: 35879 D: 69811
Ptnml(0-2): 61, 15513, 39584, 15618, 57
closes https://github.com/official-stockfish/Stockfish/pull/5335
Bench: 1336115