Don't error with zero length elem in add_to_last

[CSJCampbell]

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)
  }
}

[codecov-io]

Codecov Report

Merging #17 into master will decrease coverage by 0.12%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   84.76%   84.64%   -0.13%     
==========================================
  Files           5        5              
  Lines         302      306       +4     
==========================================
+ Hits          256      259       +3     
- Misses         46       47       +1

Impacted Files	Coverage Δ
R/flowgraph.R	99.53% <80%> (-0.47%)	⬇️

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