g_string_new_len() allocates the buffer with length
bytes. Maybe it should be obvious (wasn't to me), but
if a init argument is given, that is taken as containing
length bytes.

So,

    str = g_string_new_len (init, len);

is more like

    str = g_string_new_len (NULL, len);
    g_string_append_len (str, init, len);

and not (how I wrongly thought)

    str = g_string_new_len (NULL, len);
    g_string_append (str, init);

Fixes: 95b006c2