Modified equalizer so that previously calculated overlap statistics can be...

Modified equalizer so that previously calculated overlap statistics can be used when there happen to be non-overlapping images in the input image list. Fixes #2282.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6915 41f8697f-d340-4b68-9986-7bafba869bb8