Fix an assert when we stop the search

When signal 'stop' is raised we return bestValue
that could be still set at -VALUE_INFINITE and
this triggers an assert. Fix it by returning
a value we know for sure is not +-VALUE_INFINITE.

Reported by 平岡拓也 Hiraoka.

No functional change.

src/search.cpp

@@ -958,7 +958,7 @@ split_point_start: // At split points actual search starts from here
       // ran out of time. In this case, the return value of the search cannot
       // be trusted, and we don't update the best move and/or PV.
       if (Signals.stop || thisThread->cutoff_occurred())
-          return bestValue;
+          return value; // To avoid returning VALUE_INFINITE
 
       if (RootNode)
       {