Correctly output lowerbound/upperbound scores

fixes the lowerbound/upperbound output by avoiding
scores outside the alpha,beta bracket. Since SF search
uses fail-soft we can't simply take the returned value
as score.

closes https://github.com/official-stockfish/Stockfish/pull/4259

No functional change

src/search.h

@@ -71,6 +71,7 @@ struct RootMove {
   Value score = -VALUE_INFINITE;
   Value previousScore = -VALUE_INFINITE;
   Value averageScore = -VALUE_INFINITE;
+  Value uciScore = -VALUE_INFINITE;
   bool scoreLowerbound = false;
   bool scoreUpperbound = false;
   int selDepth = 0;