When you call MPI_Graph_create with a old_comm of size N, and pass
nnodes=(N=1), then the Nth proc is supposed to get MPI_COMM_NULL out.
The code in this base function didn't properly handle the proc(s) that
are supposed to get MPI_COMM_NULL out.
cmr=v1.7.5:reviewer=hjelmn
This commit was SVN r31145.