Avoid a needless locking in sp_search()

Only in less then 2% of cases we have a new sp->bestValue,
so check before to lock and save a costly locking
most of the times.

Patch suggested by Joona.

No functional search.

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

src/search.cpp

@@ -1759,6 +1759,8 @@ namespace {
           break;
 
       // New best move?
+      if (value > sp->bestValue) // Less then 2% of cases
+      {
           lock_grab(&(sp->lock));
           if (value > sp->bestValue && !thread_should_stop(threadID))
           {
@@ -1775,6 +1777,7 @@ namespace {
           }
           lock_release(&(sp->lock));
       }
+    }
 
     lock_grab(&(sp->lock));