Fix a subtle bug due to the StateInfo pointer became stale

There was one occurence when the StateInfo variable went
out of scope before the corresponding Position object.

This yelds to a crash. Bug was not hit before because occurs
only when using an UCI interface and not the usual benchmark.

The fix consists in copying internally the content of the
about to stale StateInfo.

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

src/uci.cpp

@@ -22,6 +22,7 @@
 //// Includes
 ////
 
+#include <cassert>
 #include <iostream>
 #include <sstream>
 #include <string>
@@ -228,6 +229,9 @@ namespace {
                 if (RootPosition.rule_50_counter() == 0)
                     RootPosition.reset_game_ply();
             }
+            // Our StateInfo st is about going out of scope,
+            // so save its content before they disappear.
+            RootPosition.setStartState(st);
         }
     }
   }
@@ -321,6 +325,8 @@ namespace {
     if (moveTime)
         infinite = true;  // HACK
 
+    assert(RootPosition.is_ok());
+
     think(RootPosition, infinite, ponder, RootPosition.side_to_move(), time,
           inc, movesToGo, depth, nodes, moveTime, searchMoves);
   }