Small code reformat in TranspositionTable::extract_pv()

In particular don't use an array of StateInfo, this
avoids a possible overflow and is in any case redundant.

Also pass as argument the pv[] array size to avoid a second
possible overflow on this one.

Fix suggested by Joona.

No functional change.

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

src/search.cpp

@@ -947,7 +947,7 @@ namespace {
             // Update PV
             rml.set_move_score(i, value);
             update_pv(ss, 0);
-            TT.extract_pv(pos, ss[0].pv);
+            TT.extract_pv(pos, ss[0].pv, PLY_MAX);
             rml.set_move_pv(i, ss[0].pv);
 
             if (MultiPV == 1)