Replace

    g_assert(err != NULL);
    error_free(err);
    err = NULL;

and variations thereof by

    error_free_or_abort(&err);

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200630090351.1247703-6-armbru@redhat.com>