currentIndex not updated correctly if list is initially empty

currentIndex not updated correctly if list is initially empty

The currentIndex shouldn't be incremented when itemCount == 0 and
also it should be default to 0 after the first item is added.

Task-number: QTBUG-21643
Change-Id: Ia9418c0cd1cd659410123394c880dfe72557fa16
Reviewed-on: http://codereview.qt-project.org/5768
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>