Fix 'bench' for Chess960 case

Fix 'bench' for Chess960 case

Now a fen file with Chess960 positions is
correctly parsed. But it is mandatory to set
"UCI_Chess960" option _before_ to call bench.

Note that this was not needed/possible before
adding the possibility to call 'bench' from
command prompt.

No functional change.

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