Correct cause by multiple dictionary for an STL collection.

Previous if the user did (implicitly or explicitly)
  - load first dictionary for a class (without a rootpcm)
  - access the TClass for that class
  - load a second dictionary (with a rootpcm) for the same class
  - Request the list of data members or the list of base class
    for that class.
then the pointer to either list was a nullptr (rather than a
pointer to the proper list as provided by the first dictionary).