When moving foreign particles to the end of the list, make sure we don't swap...

When moving foreign particles to the end of the list, make sure we don't swap with one that is a foreign particle.

src/space.c

@@ -433,7 +433,7 @@ void space_rebuild(struct space *s, double cell_max, int verbose) {
 #ifdef WITH_MPI
   /* Move non-local parts to the end of the list. */
   const int local_nodeID = s->e->nodeID;
-  for (size_t k = 0; k < nr_parts; k++)
+  for (size_t k = 0; k < nr_parts;) {
     if (cells[ind[k]].nodeID != local_nodeID) {
       cells[ind[k]].count -= 1;
       nr_parts -= 1;
@@ -453,9 +453,14 @@ void space_rebuild(struct space *s, double cell_max, int verbose) {
       ind[k] = ind[nr_parts];
       ind[nr_parts] = t;
     }
+    else {
+      /* Increment when not exchanging otherwise we need to retest "k".*/
+      k++;
+    }
+  }
 
   /* Move non-local gparts to the end of the list. */
-  for (int k = 0; k < nr_gparts; k++)
+  for (int k = 0; k < nr_gparts;) {
     if (cells[gind[k]].nodeID != local_nodeID) {
       cells[gind[k]].gcount -= 1;
       nr_gparts -= 1;
@@ -472,6 +477,11 @@ void space_rebuild(struct space *s, double cell_max, int verbose) {
       gind[k] = gind[nr_gparts];
       gind[nr_gparts] = t;
     }
+    else {
+      /* Increment when not exchanging otherwise we need to retest "k".*/
+      k++;
+    }
+  }
 
   /* Exchange the strays, note that this potentially re-allocates
      the parts arrays. */