In Mesh::GeneratePartitioning, fix a bug in the case when the number of elements is less than the number of processors.
In Mesh::GeneratePartitioning, fix a bug in the case when the
number of elements is less than the number of processors.

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem