Fixed bug which caused a bad pointer to SPICE blobs in very large cube labels....

Fixed bug which caused a bad pointer to SPICE blobs in very large cube labels.  This was related to the refactor of the IString class, References #1312.  Fixes #1388.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4959 41f8697f-d340-4b68-9986-7bafba869bb8