`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.