2.0.0: MPI_Group_intersection: malloc(0) warning with 2.0.0rc3

Trivial Python code checking the intersection of the empty group with itself.

``` python
# file: tmp.py
from mpi4py import MPI
empty = MPI.Group.Intersection(MPI.GROUP_EMPTY, MPI.GROUP_EMPTY)
assert MPI.Group.Compare(empty, MPI.GROUP_EMPTY) in [MPI.IDENT, MPI.CONGRUENT]
```

``` console
$ mpiexec -n 1 python tmp.py
malloc debug: Request for 0 bytes (group/group.c, 456)
```
