Fix Potential in TB cutoffs for NMP.

Removes the second dependency on beta and caps the return value to VALUE_TB_WIN_IN_MAX_PLY - 1

Earlier tests:

STC:
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 193632 W: 51372 L: 51326 D: 90934
Ptnml(0-2): 447, 20111, 55687, 20091, 480
https://tests.stockfishchess.org/tests/view/6486ee4465ffe077ca125bc1

LTC:
LLR: 2.97 (-2.94,2.94) <-1.75,0.25>
Total: 331758 W: 89538 L: 89624 D: 152596
Ptnml(0-2): 114, 30121, 105516, 29993, 135
https://tests.stockfishchess.org/tests/view/6489401af42a44347ed7be42

updated constant:
LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 100260 W: 27143 L: 27017 D: 46100
Ptnml(0-2): 34, 8842, 32248, 8976, 30
https://tests.stockfishchess.org/tests/view/6492fcafdc7002ce609c818c

closes: #4632
fixes: #4598

bench: 2370027

src/search.cpp

@@ -801,10 +801,9 @@ namespace {
         if (nullValue >= beta)
         {
             // Do not return unproven mate or TB scores
-            if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
-                nullValue = beta;
+            nullValue = std::min(nullValue, VALUE_TB_WIN_IN_MAX_PLY-1);
 
-            if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
+            if (thisThread->nmpMinPly || depth < 14)
                 return nullValue;
 
             assert(!thisThread->nmpMinPly); // Recursive verification is not allowed