The `Comm::Free` method may be called more than once by applications.
Although some MPI vendors document that `MPI_Comm_free` sets the
given handle to `MPI_COMM_NULL`, this does not always happen in
practice.  Set the handle to `MPI_COMM_NULL` explicitly to avoid
calling `MPI_Comm_free` more than once for the same handle.

Without this, when compiled with PGI and executed with TAU profiling,
a double-free occurs and causes memory to be corrupted.