Alter how \str_substr:nnn treats a zero argument

Now '0' has the opposite behaviour as an empty argument.
That means that the ranges {}{n} and {-n}{} have n
characters for n from 0 to the length N of the string.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4445 de43f980-851b-0410-b2f7-c40aca1f87e0