When calling MPI_Topo_test, explicitly deal with MPI_COMM_NULL as it is not a legal value. Although it is what is return for leftovers process when creating cartesian grid or graphs.
Merge branch 'develop' of github.com:boostorg/mpi into nulltopo
Merge pull request #20 from aminiussi/nulltopo

MPI_COMM_NULL is not a valid argument for MPI_Topo_test.
But it is used to indicated leftover processes when creating graph communicator or cartesians grids (for exampl, when you create a 2X2 grid from a 5 process communicator).

So we need to deal with it as a special case.