[expt] fixes negative mpi tag

cpp/particles/particles_distr_mpi.hpp

@@ -148,11 +148,14 @@ protected:
         // some environments need the tag to be small
         // see https://www.intel.com/content/www/us/en/developer/articles/technical/large-mpi-tags-with-the-intel-mpi.html
         // TODO: is there a better way of achieving this result?
-        const int new_mpi_tag = mpi_tag_input % maximum_MPI_tag_value;
+        int new_mpi_tag = mpi_tag_input % maximum_MPI_tag_value;
+        if (new_mpi_tag < 0)
+            new_mpi_tag = -new_mpi_tag; // no idea if this is reasonable...
         if (new_mpi_tag >= maximum_MPI_tag_value || new_mpi_tag < 0)
         {
-            DEBUG_MSG("new_mpi_tag is %d, MPI_TAG_UB is %d, 2**20 is 1048576\n",
+            DEBUG_MSG("new_mpi_tag is %d, MPI_TAG_UB is %d, mpi_tag_input is %d, 2**20 is 1048576\n",
                     new_mpi_tag,
+                    mpi_tag_input,
                     MPI_TAG_UB);
             assert(false);
         }