Fix escape key when no reveal is shown

If you try to press escape key when no reveal modal is shown; an error is thrown. Fixed.

Line 107/109 :
var open_modal = $('[data-reveal].open'), // [] if no reveal modal shown
     settings = open_modal.data('reveal-init'); // undefined if open_modal == []
if ( settings && event.which === 27  && settings.close_on_esc) { // settings.close_on_esc throw Error.