second round of making per-atom allocations consistent.

now all sections that dimension per-atom local arrays to atom->nmax
will also *test* for atom->nmax and thus we avoid potential segfaults
in all cases where atom->nlocal or atom->nlocal+atom->nghost are 0.
it doesn't make sense to test for these if the arrays are dimensioned
to atom->nmax afterwards anyway.

src/compute_cluster_atom.cpp

@@ -104,7 +104,7 @@ void ComputeClusterAtom::compute_peratom()
 
   // grow clusterID array if necessary
 
-  if (atom->nlocal+atom->nghost > nmax) {
+  if (atom->nmax > nmax) {
     memory->destroy(clusterID);
     nmax = atom->nmax;
     memory->create(clusterID,nmax,"cluster/atom:clusterID");