Allow AccumulatorStack::size to point to one past the end

this is guaranteed to be correct since we access the last element with `size - 1`

closes https://github.com/official-stockfish/Stockfish/pull/6368

fixes https://github.com/official-stockfish/Stockfish/issues/6367

no functional change

src/nnue/nnue_accumulator.cpp

@@ -75,7 +75,7 @@ void AccumulatorStack::reset() noexcept {
 }
 
 void AccumulatorStack::push(const DirtyPiece& dirtyPiece) noexcept {
-    assert(size + 1 < accumulators.size());
+    assert(size < accumulators.size());
     accumulators[size].reset(dirtyPiece);
     size++;
 }