Fix old Glaurung bug related to search logging

When we log best and ponder move to a file before to
return from think we change the position. If position is
then not resended by GUI, as for manual user input we got
an error:

justinb@malibu:~$ stockfish
Stockfish 2.0 JA 64bit by Tord Romstad, Marco Costalba, Joona Kiiski
setoption name Use Search Log value true
go depth 1
info depth 1
info depth 1 seldepth 1 multipv 1 score cp 72 time 59 nodes 20 nps 338 pv g1f3
info depth 2
info depth 2 seldepth 2 multipv 1 score cp 12 time 59 nodes 44 nps 745 pv g1f3 g8f6
info nodes 84 nps 1423 time 59
bestmove g1f3 ponder g8f6
go depth 1
info depth 1 score mate 0
info nodes 87 nps 0 time 0
bestmove (none) ponder (none)

Bug spotted and fixed by UncombedCoconut.

No functional change.

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

src/search.cpp

@@ -513,6 +513,9 @@ bool think(Position& pos, bool infinite, bool ponder, int time[], int increment[
               << move_to_san(pos, ponderMove) // Works also with MOVE_NONE
               << endl;
 
+      // Return from think() with unchanged position
+      pos.undo_move(bestMove);
+
       LogFile.close();
   }