Only finalize the centre of mass calculation if the mass is non-zero

src/engine.c

@@ -4469,8 +4469,7 @@ void engine_init(struct engine *e, struct space *s,
    * do this once we've made at least one call to engine_entry_affinity and
    * maybe numa_node_of_cpu(sched_getcpu()), even if the engine isn't already
    * pinned. */
-  if (with_aff)
-    engine_unpin();
+  if (with_aff) engine_unpin();
 #endif
 
   if (with_aff) {

src/statistics.c

@@ -301,9 +301,11 @@ void stats_collect(const struct space *s, struct statistics *stats) {
  */
 void stats_finalize(struct statistics *stats) {
 
-  stats->centre_of_mass[0] /= stats->mass;
-  stats->centre_of_mass[1] /= stats->mass;
-  stats->centre_of_mass[2] /= stats->mass;
+  if (stats->mass > 0.) {
+    stats->centre_of_mass[0] /= stats->mass;
+    stats->centre_of_mass[1] /= stats->mass;
+    stats->centre_of_mass[2] /= stats->mass;
+  }
 }
 
 /**