Fix error while setting slider value from text input.

Hi, I made a small fix. It's preventing from "Uncaught Error Invalid input value 'x' passed in" error, when you have text input connected with slider, for instance:

    <input type="text" class="slider form-control" data-bind="sliderValue: selectedValue"/>
    <input type="text" data-bind="value: selectedValue">

And you're trying to manipulate slider value by typing number into this text input.