Fix removal (assignment to nil) of elements in stdnse.output_table.

There was a reverse table that was meant to map keys to their position
in the order table, to make it easy to table.remove an entry from order.
But removing something from order would shift the indices of all
following elements by 1, and those elements were not updated
correspondingly in the reverse table. Instead, do a linear lookup for
the the element to remove from order, after checking that the element
exists at all.

http://seclists.org/nmap-dev/2012/q3/905

nselib/stdnse.lua

@@ -1122,7 +1122,6 @@ end
 -- @return An ordered table.
 function output_table ()
   local t = {}
-  local reverse = {}
   local order = {}
   local function iterator ()
     for i, key in ipairs(order) do
@@ -1131,19 +1130,19 @@ function output_table ()
   end
   local mt = {
     __newindex = function (_, k, v)
-      if reverse[k] then
-        rawset(t, k, v)
-        if v == nil then
-          table.remove(order, reverse[k])
-          reverse[k] = nil
+      if t[k] == nil then
+        -- New key?
+        table.insert(order, k)
+      elseif v == nil then
+        -- Deleting an existing key?
+        for i, key in ipairs(order) do
+          if key == k then
+            table.remove(order, i)
+            break
+          end
         end
-      else
-        if v ~= nil then
-          table.insert(order, k)
-          reverse[k] = #order
-        end
-        rawset(t, k, v)
       end
+      rawset(t, k, v)
     end,
     __index = function (_, k)
       return t[k]