Fix out-of-bound array access printing ponder move

It is possible that we won't have a ponder move if our PV
is too short.  In that case, just don't print a ponder move.

No functional change
I know I am a bit nitpicking here, but could you please commit addinga caouple of blank lines to improve readibility?

 sync_cout << "bestmove " << UCI::format_move(RootMoves[0].pv[0], RootPos.is_chess960());

 if (RootMoves[0].pv.size() > 1)
 std::cout << " ponder " << UCI::format_move(RootMoves[0].pv[1], RootPos.is_chess960());

 std::cout << sync_endl;