Don't error with zero length elem in add_to_last

[gaborcsardi]

Cf. MangoTheCat/cyclocomp#17

The following expression from Rmpi raises an error due to breaks_tail seeing a zero length breaks.

 function (dellog = TRUE, comm = 1) {
  if (mpi.comm.size(comm) < 2) {
    err <- paste("It seems no slaves running on comm", comm)
    stop(err)
  }
  mpi.bcast.cmd(cmd = break, rank = 0, comm = comm)
  if (.Platform$OS != "windows") {
    if (dellog && mpi.comm.size(0) < mpi.comm.size(comm)) {
      tmp <- paste(Sys.getpid(), "+", comm, sep = "")
      logfile <- paste("*.", tmp, ".*.log", sep = "")
      if (length(system(paste("ls", logfile), TRUE, ignore.stderr = TRUE)) >= 
          1)
        system(paste("rm", logfile))
    }
  }
  if (comm > 0) {
    if (is.loaded("mpi_comm_disconnect")) 
      mpi.comm.disconnect(comm)
    else mpi.comm.free(comm)
  }
}

[gaborcsardi]

Hmmm, if this happens because of a bug in cyclocomp, then we could fix the bug, I guess. FWIW there is no error on Rmpi currently, so I am going to close this now.