TargetInfoWidget no longer triggers a segfault if the spice labels do no...

TargetInfoWidget no longer triggers a segfault if the spice labels do no contain proper target info.  Fixes #4954.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce@7891 41f8697f-d340-4b68-9986-7bafba869bb8