The default currentIndex for an empty PathView is 0, and except when
there is no model or an empty model setCurrentIndex won't allow a
currentIndex outside the valid index range for the model. This changes
the wrapping for negative numbers in setCurrentIndex so that it is
consistent with decrementCurrentIndex and forces the currentIndex to
0 if there are no model items.

Task-number: QTBUG-21316
Change-Id: Id4d4d78e9832d05baf8a9d148e7f81ee89c9bc61
Reviewed-by: Martin Jones <martin.jones@nokia.com>