Offset pv[] always from 0

We don't need to offset from current ply.

Also rewritten a bit update_pv()

No functional change.

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

src/search.h

@@ -60,7 +60,7 @@ struct SearchStack {
   Value eval;
   bool skipNullMove;
 
-  void init(int ply);
+  void init();
   void initKillers();
 };