`mask` returns invalid result for ISBN with 0 idx

[mgradalska]

When I'm trying to mask 9786129999999 ISBN

mask('9786129999999')

it results in string with doubled separator:

'978-612--999999-9'

It doesn't look correct to me. I think the culprit here is that in this part of code

isbnlib/isbnlib/_msk.py

Line 62 in ddc62f2

[group, ib[cur + 1:cur + idx + 1], ib[cur + idx + 1:-1], check])

the second entry in the list is ib[6:6] (since idx is 0) and it returns an empty string.

[hans-fritz-pommes]

This is happening because the data for masking was not updated for a long time.
@xlcnd had no activity on github for a long time now. I have created a fork and am now trying to maintain it.
https://github.com/hans-fritz-pommes/isbnlib
You can install it with pip install isbnlib2.
Maybe remove isbnlib before.
The module name is the same - your code will run the same way.