Fix incorrect initialization

Fixes https://github.com/official-stockfish/Stockfish/issues/6505

Missing initialization seemingly resulting in side effects, as discussed in the issue.

Credit to Sopel for spotting the bug.

PR used as a testcase for CoPilot, doing the right thing https://github.com/official-stockfish/Stockfish/pull/6478#discussion_r2655467218

closes https://github.com/official-stockfish/Stockfish/pull/6511

No functional change

src/history.h

@@ -105,10 +105,10 @@ struct DynStats {
         data = make_unique_large_page<T[]>(size);
     }
     // Sets all values in the range to 0
-    void clear_range(int value, size_t threadIdx) {
+    void clear_range(int value, size_t threadIdx, size_t numaTotal) {
-        size_t start = threadIdx * SizeMultiplier;
+        size_t start = uint64_t(threadIdx) * size / numaTotal;
         assert(start < size);
-        size_t end = std::min(start + SizeMultiplier, size);
+        size_t end = threadIdx + 1 == numaTotal ? size : uint64_t(threadIdx + 1) * size / numaTotal;
 
         while (start < end)
             data[start++].fill(value);

src/search.cpp

@@ -582,8 +582,8 @@ void Search::Worker::clear() {
     captureHistory.fill(-689);
 
     // Each thread is responsible for clearing their part of shared history
-    sharedHistory.correctionHistory.clear_range(0, numaThreadIdx);
+    sharedHistory.correctionHistory.clear_range(0, numaThreadIdx, numaTotal);
-    sharedHistory.pawnHistory.clear_range(-1238, numaThreadIdx);
+    sharedHistory.pawnHistory.clear_range(-1238, numaThreadIdx, numaTotal);
 
     ttMoveHistory = 0;