Support in-memory initialization

When range input elements are created in memory, the value for
`maxHandleX` will be zero. This causes the value reported by the initial
"slide" event to be NaN.

    $('<input type="range" min="0" max="1">').rangeslider({
      onSlide: function(pos, val) {
        // Number.isNaN(val) === true;
      }
    });

Modify the value calculation to return 0 when `maxHandleX` is 0.