Skip evaluate() call after a null move

Reuse the evaluation of the parent with inverted sign and
set margin to zero (this is an hack!).

This is done only in qsearch where almost 15% of calls are
from a null move. In normal search the number of nodes where

(ss-1)->currentMove == MOVE_NULL

is almost zero and so there is no need of using this trick.

The big advantage of this patch is a speed-up due to skipped
evaluate() calls, that are very costly.

Functionality is of course affected and we will need to proper
test it later. For now we just register a 3-4% speed up.

Suggested by Hongzhi Cheng.

bench: 5051328

src/search.cpp

@@ -1154,6 +1154,11 @@ split_point_start: // At split points actual search starts from here
             if (ss->staticEval == VALUE_NONE || ss->evalMargin == VALUE_NONE) // Due to a race
                 ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
         }
+        else if ((ss-1)->currentMove == MOVE_NULL)
+        {
+            ss->staticEval = bestValue = -(ss-1)->staticEval;
+            ss->evalMargin = VALUE_ZERO; // Hack, we really don't know the value
+        }
         else
             ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);