message: mpidummy: Add MPI_OP_NULL constant
  Use value `0` for consistency with `MPI_DATATYPE_NULL` and increment
  the index of other "op" constants.
diff: @@ -82,20 +82,21 @@ typedef int MPI_Op;
  #define MPI_ANY_SOURCE 0
  #define MPI_ANY_TAG 0
  -#define MPI_MAX 0
  -#define MPI_MIN 1
  -#define MPI_SUM 2
  -#define MPI_PROD 3
  -#define MPI_LAND 4
  -#define MPI_BAND 5
  -#define MPI_LOR 6
  -#define MPI_BOR 7
  -#define MPI_LXOR 8
  -#define MPI_BXOR 9
  -#define MPI_MAXLOC 10
  -#define MPI_MINLOC 11
  -#define MPI_REPLACE 12
  -#define MPI_NO_OP 13
  +#define MPI_OP_NULL 0
  +#define MPI_MAX 1
  +#define MPI_MIN 2
  +#define MPI_SUM 3
  +#define MPI_PROD 4
  +#define MPI_LAND 5
  +#define MPI_BAND 6
  +#define MPI_LOR 7
  +#define MPI_BOR 8
  +#define MPI_LXOR 9
  +#define MPI_BXOR 10
  +#define MPI_MAXLOC 11
  +#define MPI_MINLOC 12
  +#define MPI_REPLACE 13
  +#define MPI_NO_OP 14
  #define MPI_MAX_PROCESSOR_NAME 32
  