Fix validate_slength, arg.length should be args[0].length

During a puppet run an error will be thrown and a puppet run will fail completely (when using validate_slength):
undefined local variable or method `arg' for #<Puppet::Parser::Scope:0x7f243c236948>
Merge pull request #169 from hdeheer/patch-1

Fix validate_slength, arg.length should be args[0].length