Initialize futilityMargin in EvalInfo c'tor

This is less prone to bugs because now it's up to the
compiler don't forget this important initialization.

No functional change.

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

src/evaluate.h

@@ -47,6 +47,8 @@ class Position;
 
 struct EvalInfo {
 
+  EvalInfo() : futilityMargin(Value(0)) {}
+
   // Middle game and endgame evaluations
   Score value;