Trying to select a control point within an image that has no control points...

Trying to select a control point within an image that has no control points gives a warning instead of crashing. Fixes #2210.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6193 41f8697f-d340-4b68-9986-7bafba869bb8