Do not copy master position in split()

A pointer is enough because after a split point has been
setup master and slaves thread end up calling sp_search() or
sp_search_pv() and here a full copy of split point position is
done again, note that even master does another copy (of itself)
and this is done before any do_move() call so that master Position
is never updated between split() and sp_search().

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

src/uci.cpp

@@ -314,7 +314,7 @@ namespace {
 
     string token;
     int depth, tm, n;
-    Position pos = RootPosition;
+    Position pos(RootPosition);
 
     if (!(uip >> depth))
         return;