Fix some 'possible loss of data' warnings

Patch by Maxim Masiutin

closes https://github.com/official-stockfish/Stockfish/pull/4440

No functional change

src/syzygy/tbprobe.cpp

@@ -995,13 +995,19 @@ uint8_t* set_sizes(PairsData* d, uint8_t* data) {
     d->lowestSym = (Sym*)data;
     d->base64.resize(d->maxSymLen - d->minSymLen + 1);
 
+    // See https://en.wikipedia.org/wiki/Huffman_coding
     // The canonical code is ordered such that longer symbols (in terms of
     // the number of bits of their Huffman code) have lower numeric value,
     // so that d->lowestSym[i] >= d->lowestSym[i+1] (when read as LittleEndian).
     // Starting from this we compute a base64[] table indexed by symbol length
     // and containing 64 bit values so that d->base64[i] >= d->base64[i+1].
-    // See https://en.wikipedia.org/wiki/Huffman_coding
-    for (int i = d->base64.size() - 2; i >= 0; --i) {
+
+    // Implementation note: we first cast the unsigned size_t "base64.size()" 
+    // to a signed int "base64_size" variable and then we are able to subtract 2,
+    // avoiding unsigned overflow warnings.
+
+    int base64_size = static_cast<int>(d->base64.size());
+    for (int i = base64_size - 2; i >= 0; --i) {
         d->base64[i] = (d->base64[i + 1] + number<Sym, LittleEndian>(&d->lowestSym[i])
                                          - number<Sym, LittleEndian>(&d->lowestSym[i + 1])) / 2;
 
@@ -1012,10 +1018,10 @@ uint8_t* set_sizes(PairsData* d, uint8_t* data) {
     // than d->base64[i+1] and given the above assert condition, we ensure that
     // d->base64[i] >= d->base64[i+1]. Moreover for any symbol s64 of length i
     // and right-padded to 64 bits holds d->base64[i-1] >= s64 >= d->base64[i].
-    for (size_t i = 0; i < d->base64.size(); ++i)
+    for (int i = 0; i < base64_size; ++i)
         d->base64[i] <<= 64 - i - d->minSymLen; // Right-padding to 64 bits
 
-    data += d->base64.size() * sizeof(Sym);
+    data += base64_size * sizeof(Sym);
     d->symlen.resize(number<uint16_t, LittleEndian>(data)); data += sizeof(uint16_t);
     d->btree = (LR*)data;