Fixed qnet bug when creating a new network. Program would seg fault when...

Fixed qnet bug when creating a new network.  Program would seg fault when left-clicking on a cube when the control network was empty.  Fixes #952.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4590 41f8697f-d340-4b68-9986-7bafba869bb8