mpi/src: properly deal with unknown datatypes

mpi/src/starpu_mpi.c

@@ -203,6 +203,7 @@ static struct _starpu_mpi_req *_starpu_mpi_isend_irecv_common(starpu_data_handle
 
 	req->request_type = request_type;
 	req->user_datatype = -1;
+	req->datatype = 0;
 	req->count = -1;
 	req->data_handle = data_handle;
 	req->srcdst = srcdst;

mpi/src/starpu_mpi_datatype.c

@@ -227,6 +227,6 @@ char *_starpu_mpi_datatype(MPI_Datatype datatype)
      if (datatype == MPI_INTEGER4) return "MPI_INTEGER4";
      if (datatype == MPI_INTEGER8) return "MPI_INTEGER8";
      if (datatype == MPI_PACKED) return "MPI_PACKED";
-     if (datatype == -1) return "Unknown datatype";
+     if (datatype == 0) return "Unknown datatype";
      return "User defined MPI Datatype";
 }