Fix issue with interpolating more than one path segment backwards.

Change-Id: I463010ed63d41be80db96e8306aef8caa3863ccf
Reviewed-on: http://codereview.qt-project.org/5821


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>