`tolist` bug revealed by Uproot 5 testing

[jpivarski]

Version of Awkward Array

HEAD

Description and code to reproduce

In scikit-hep/uproot5#620, a test named test_gohep_output_file is failing in tolist:

    with uproot.open(skhep_testdata.data_path("uproot-issue413.root"))[
        "mytree"
    ] as tree:
        assert tree["Str"].array().tolist()

                                   ^^^^^^^^

It looks like an Awkward bug, to be fixed here. (Uproot has a work-around, so we won't see it again unless we actively look for it. Hence this bug report.)