Fix escape key when no reveal is shown

If you try to press Escape when no reveal modal is shown, an error is thrown : "Uncaught TypeError: Cannot read property 'close_on_esc' of undefined ".
Because at the line 107 : "var open_modal = $('[data-reveal].open')" will return [], "settings = open_modal.data('reveal-init')" will return "undefined" and "settings.close_on_esc" will throw the error.