No need to do anything for sendcount==0 case

This commit was SVN r2904.

src/mpi/c/alltoall.c

@@ -47,6 +47,13 @@ int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
       OMPI_ERRHANDLER_CHECK(err, comm, err, FUNC_NAME);
     }
 
+    /* If the sendcount is 0, since everyone gave the same value, then
+       we don't need to do anything */
+
+    if (0 == sendcount) {
+        return OMPI_SUCCESS;
+    }
+
     /* Invoke the coll component to perform the back-end operation */
 
     err = comm->c_coll.coll_alltoall(sendbuf, sendcount, sendtype,