Added check to the Vims camera model to handle cubes which have no pixel...

Added check to the Vims camera model to handle cubes which have no pixel centers intersecting the planet.  Fixes #1306

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4907 41f8697f-d340-4b68-9986-7bafba869bb8