Fix a possible 'Division by zero'

In case a book entry has 'count' field set to 0
we crash. Spotted by Clang's static analyzer.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.cpp

@@ -420,7 +420,7 @@ Move Book::probe(const Position& pos, const string& fName, bool pickBest) {
       // Choose book move according to its score. If a move has a very
       // high score it has higher probability to be choosen than a move
       // with lower score. Note that first entry is always chosen.
-      if (   (RKiss.rand<unsigned>() % sum < e.count)
+      if (   (sum && RKiss.rand<unsigned>() % sum < e.count)
           || (pickBest && e.count == best))
           move = Move(e.move);
   }