load_pairs not working correctly with GRIB1 data

[pp-mo]

Example...

>>> filename = "...iris-test-data/test_data/GRIB/grib1_second_order_packing/GRIB_00008_FRANX01"
>>> from iris_grib import _load_generate
>>> fields_iter = _load_generate(filename)
>>> fields = list(fields_iter)
>>> fields[0].qqq
Segmentation fault

Originally looked like a problem with (deferred) data.
Seen when cubes = [cube for cube, field in iris_grib.load_pairs_from_fields(fields)]

But now this looks different -- ??maybe? problem with repr when generating an exception ?