debugger: Freeing a communicator causes the debugging code to segfault

[RaymondMichael]

In dll_mpich.c:rebuild_communicator_list(), when a previous communicator has been freed and the code is freeing its tracker in the debugging code, it calls group_decref() to release the communicator's group. find_or_create_group() only ever returns a NULL group, and so the group_decref() function is bound to segfault when it tries touching the NULL group.

[hzhou]

Thanks for digging up some never finished code ;-)