[QQC2 Slider Style] Fix wrong handle positioning when initial value is 1

Summary:
Old code was not reading value properly when initial value was 1, misplacing the handle as a result.

BUG: 405471

Test Plan:
Tested by running qmlscene with:

```
import QtQuick.Controls 2.5

Slider {
    from: 1
    to: 5
    stepSize: 1
    //orientation: Qt.Vertical
}
```
Tested both in horizontal and vertical orientation.

You can also test this in the Mouse KCM, with the Pointer Speed slider.

Reviewers: mart, #plasma, ngraham, broulik

Reviewed By: mart, #plasma, ngraham

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21075