added check to prevent to free MPI_COMM_SELF or MPI_COMM_WORLD

include/ALL_Histogram.hpp

@@ -391,7 +391,10 @@ template<class T, class W> void ALL_Histogram_LB<T,W>::balance(int step)
     MPI_Barrier(this->global_comm);
     // free created communicators
     for (int i = 0; i < 6; ++i)
+    {
+        if (communicators.at(i) != MPI_COMM_SELF && communicators.at(i) != MPI_COMM_WORLD)
             MPI_Comm_free(&(communicators.at(i)));
+    }
 
 }