BUG: Prevent LevelTracingEffect crash when the user clicks on a pixel at the edge of the image (http://na-mic.org/Mantis/view.php?id=3470)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22671 3bd1e089-480b-0410-8dfb-8563597acbee