Fixed issue with segfault occuring when right clicking a cnet in the project...

Fixed issue with segfault occuring when right clicking a cnet in the project tree when thre is only 1 cnet imported and no images imported. Fixes #5160.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce@8254 41f8697f-d340-4b68-9986-7bafba869bb8