creating ompi_mpi_comm_null, since the destructor of ompi_mpi_comm_null will
decrease the reference counter of ompi_mpi_group_null twice according to the
last fix of Mohamad.
Added also a lengthy comment in ompi_comm_finalize about why we do
not decrease the reference counters for ompi_mpi_comm_null,
ompi_mpi_group_null etc. for the parent
communicator, although we do increase it in ompi_comm_init
This commit was SVN r14210.
188f770d94