issue when trying to access pointer of zero length vector

[tdhock]

there could be zero labels in FLOPART but that causes UBSAN issues https://www.stats.ox.ac.uk/pub/bdr/memtests/gcc-UBSAN/FLOPART/tests/testthat.Rout because the current code does

&label_type_vec[0], &label_start_vec[0], &label_end_vec[0], label_count,

which means accessing the first (non-existing) element, then getting its pointer.
RcppCore/Rcpp#1308

maybe we can use one of these methods to get the pointer? https://github.com/RcppCore/Rcpp/pull/1310/files#diff-ea8708322594fed31000f9ccae6b5a9cb7ee93ef33468b392b212f8733bbc0a9 get() ?

[tdhock]

probably coming from this test

test_that("can start up", {
  seg.mean.vec <- c(11, 3)
  set.seed(1)
  count <- unlist(lapply(seg.mean.vec, function(m)rpois(10, m)))
  N <- length(count)
  cov.df <- data.frame(
    chromStart=seq(0, N-1),
    chromEnd=seq(1, N),
    count)
  result.list <- FLOPART::FLOPART(cov.df, penalty=5)
  computed.segs <- result.list[["segments_dt"]]
  expected.segs <- data.table(
    chromStart=c(0,10),
    chromEnd=c(10,20),
    status=c("peak", "background"),
    mean=c(mean(count[1:10]),mean(count[11:20])))
  expect_equal(computed.segs, expected.segs)
})