qsearch: prevent bestValue from going down

The bestValue can sometimes go down. This happens 2% of the time or so.
This fix stops it from decreasing.

Failed gainer STC:
LLR: -2.94 (-2.94,2.94) <0.00,2.00>
Total: 146176 W: 37930 L: 37976 D: 70270
Ptnml(0-2): 480, 17422, 37366, 17304, 516
https://tests.stockfishchess.org/tests/view/6953be19572093c1986da66a

Passed Non-regression LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 257796 W: 65662 L: 65683 D: 126451
Ptnml(0-2): 164, 28247, 72087, 28246, 154
https://tests.stockfishchess.org/tests/view/69554ff0d844c1ce7cc7e333

closes official-stockfish#6520
fixes official-stockfish#6519

Bench: 2477446

Author: jake-ciolek

AUTHORS

@@ -111,6 +111,7 @@ Hongzhi Cheng
 Ivan Ivec (IIvec)
 Jacques B. (Timshel)
 Jake Senne (w1wwwwww)
+Jakub Ciolek (jake-ciolek)
 Jan Ondruš (hxim)
 Jared Kish (Kurtbusch, kurt22i)
 Jarrod Torriero (DU-jdto)

src/search.cpp

@@ -1653,7 +1653,7 @@ Value Search::Worker::qsearch(Position& pos, Stack* ss, Value alpha, Value beta)
                 // we can prune this move.
                 if (!pos.see_ge(move, alpha - futilityBase))
                 {
-                    bestValue = std::min(alpha, futilityBase);
+                    bestValue = std::max(bestValue, std::min(alpha, futilityBase));
                     continue;
                 }
             }