Initialize killers at ss+2 also in root_search()

After 4955 games:
Mod - Orig: 786 - 768 - 3401 +1 ELO

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

src/search.cpp

@@ -702,6 +702,7 @@ namespace {
 
     // Step 1. Initialize node (polling is omitted at root)
     ss->currentMove = ss->bestMove = MOVE_NONE;
+    (ss+2)->killers[0] = (ss+2)->killers[1] = (ss+2)->mateKiller = MOVE_NONE;
 
     // Step 2. Check for aborted search (omitted at root)
     // Step 3. Mate distance pruning (omitted at root)