Do not return unproven mate scores from null move search

Causes very small functional change which is not observable with
our usual set of test positions.

However change is observable fx. with following position:
4k3/3r4/5Q2/6K1/8/8/8/8 w - - 0 1
go depth 24

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

diff --git a/src/search.cpp b/src/search.cpp
index f5552e7887dcd4e03a3a243a747193917e0cbece..67c98709521ae5d63707a56d39a2f63b2a010d41 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1378,6 +1378,10 @@ namespace {
 
         if (nullValue >= beta)
         {
+            // Do not return unproven mate scores
+            if (nullValue >= value_mate_in(PLY_MAX))
+                nullValue = beta;
+
             if (depth < 6 * OnePly)
                 return nullValue;