Add a count guard check for slice

Author: José Valim

lib/elixir/lib/enum.ex

@@ -1312,11 +1312,11 @@ defmodule Enum do
 
   def slice(_coll, _start, 0), do: []
 
-  def slice(coll, start, count) when is_list(coll) and start >= 0 do
+  def slice(coll, start, count) when is_list(coll) and start >= 0 and count >= 0 do
     do_slice(coll, start, count)
   end
 
-  def slice(coll, start, count) when start >= 0 do
+  def slice(coll, start, count) when start >= 0 and count >= 0 do
     { _, _, list } = Enumerable.reduce(coll, { start, count, [] }, fn
       _entry, { start, count, _list } when start > 0 ->
         { start-1, count, [] }

lib/elixir/lib/string.ex

@@ -822,11 +822,11 @@ defmodule String do
     end
   end
 
-  def slice(string, start, len) when start >= 0 do
+  def slice(string, start, len) when start >= 0 and len >= 0 do
     do_slice(next_grapheme(string), start, start + len - 1, 0, "")
   end
 
-  def slice(string, start, len) when start < 0 do
+  def slice(string, start, len) when start < 0 and len >= 0 do
     real_start_pos = do_length(next_grapheme(string)) - abs(start)
     case real_start_pos >= 0 do
       true -> do_slice(next_grapheme(string), real_start_pos, real_start_pos + len - 1, 0, "")