Fix helix reselection code for designs with non-consecutive helices

[UnHumbleBen]

https://github.com/UC-Davis-molecular-computing/scadnano/blob/f3dcc69202a1578ee5bdd8969b368d6ee9763ba1/lib/src/reducers/load_dna_file_reducer.dart#L59-L63

Creating this issue as a reminder.

TODO: Create an example where this code would break