Halve king eval margin

But still keep the same original
margin for score.

Passed both short TC test
LR: 2.95 (-2.94,2.94)
Total: 3710 W: 845 L: 726 D: 2139

And long TC
LLR: 2.95 (-2.94,2.94)
Total: 57859 W: 10939 L: 10532 D: 36388

bench: 4769737

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

src/evaluate.cpp

@@ -767,7 +767,7 @@ Value do_evaluate(const Position& pos, Value& margin) {
         // be very big, and so capturing a single attacking piece can therefore
         // result in a score change far bigger than the value of the captured piece.
         score -= KingDanger[Us == Search::RootColor][attackUnits];
-        margins[Us] += mg_value(KingDanger[Us == Search::RootColor][attackUnits]);
+        margins[Us] += mg_value(KingDanger[Us == Search::RootColor][attackUnits]) / 2;
     }
 
     if (Trace)