Segfault when demultiplexing without a UMI, only when using GH Actions build

[olliecheng]

Likely due to different compiler behaviour for UB.

std::string get_umi(...) {
  ...

  int umi_start, umi_length;
  std::string umi_pad = "";
  umi_length = search_pattern[umi_index].second.length();
  
  if (umi_index == -1) {
    return ""; // protocol does not have UMI
  }

  ...
}

When umi_index is negative, this is UB.

Will roll out a patch and tests soon