second try to implement changes suggested in issue #888

In src/rcb.cpp:460 there is an if (smaller > largest).
now if we have one particle you will see that lo[] = hi[] and because
of this smaller == largest == 0 for all values of dim. This causes
this particular part of the code to never be run. In particular the
memcpy inside this if is never executed. This causes an unitialized
memory access in line 472. Additionally, dim is initialized with -1
and thus the accesses in 484 and 485 are problematic. Additionally,
valuehalf_select is never initialized either.

closes #888

src/balance.cpp

@@ -28,6 +28,7 @@
 #include "rcb.h"
 #include "irregular.h"
 #include "domain.h"
+#include "neighbor.h"
 #include "force.h"
 #include "update.h"
 #include "group.h"
@@ -643,6 +644,19 @@ int *Balance::bisection(int sortflag)
   double *shrinklo = &shrinkall[0];
   double *shrinkhi = &shrinkall[3];
 
+  // ensure that that the box has at least some extent.
+  const double nproc_rt = domain->dimension == 3 ?
+                          cbrt(static_cast<double>(comm->nprocs)) :
+                          sqrt(static_cast<double>(comm->nprocs));
+  const double min_extent = ceil(nproc_rt)*neighbor->skin;
+  for (int i = 0; i < domain->dimension; i++) {
+    if (shrinkall[3+i]-shrinkall[i] < min_extent) {
+      const double mid = 0.5*(shrinkall[3+i]+shrinkall[i]);
+      shrinkall[3+i] = std::min(mid + min_extent*0.5, boxhi[i]);
+      shrinkall[i]   = std::max(mid - min_extent*0.5, boxlo[i]);
+    }
+  }
+
   // invoke RCB
   // then invert() to create list of proc assignments for my atoms
   // NOTE: (3/2017) can remove undocumented "old" option at some point

src/rcb.cpp

@@ -243,7 +243,7 @@ void RCB::compute(int dimension, int n, double **x, double *wt,
     // dotmark_select = dot markings in that dimension
 
     int dim_select = -1;
-    double largest = 0.0;
+    double largest = -1.0;
 
     for (dim = 0; dim < dimension; dim++) {