allow chop to take an empty string

[bicycle1885]

The following behavior contradicts the docstring of chop. So, I fixed the behavior in this pull request.

julia> chop("")
ERROR: BoundsError: attempt to access ""
  at index [1]
Stacktrace:
 [1] nextind(::String, ::Int64, ::Int64) at ./strings/basic.jl:521
 [2] #chop#328(::Int64, ::Int64, ::typeof(chop), ::String) at ./strings/util.jl:99
 [3] chop(::String) at ./strings/util.jl:99
 [4] top-level scope at REPL[1]:1

help?> chop
search: chop chomp readchomp chown chmod Cshort eachrow eachcol

  chop(s::AbstractString; head::Integer = 0, tail::Integer = 1)

  Remove the first head and the last tail characters from s. The
  call chop(s) removes the last character from s. If it is
  requested to remove more characters than length(s) then an empty
  string is returned.

  Examples
  ≡≡≡≡≡≡≡≡≡≡

  julia> a = "March"
  "March"

  julia> chop(a)
  "Marc"

  julia> chop(a, head = 1, tail = 2)
  "ar"

  julia> chop(a, head = 5, tail = 5)
  ""

[StefanKarpinski]

Obviously needs to be fixed but I worry that this may just be bandaiding the real problem, namely that nextind("", 1, 0) should probably work. cc @bkamins; I believe this was tightened from my original.

[StefanKarpinski]

Have merged this as the simplest most conservative fix that can and should be backported.