Let charset arg to stdnse.generate_random_string be a string

The charset argument was intended to be a table, since Lua doesn't allow
indexing strings with []. Unfortunately, irc-brute and tftp-enum passed
a string instead, which meant that the output was always the empty
string. This change allows both formats, and fixes those scripts.

irc-brute and tftp-enum had been broken since r33632 (no released
version of Nmap)

nselib/stdnse.lua

@@ -252,7 +252,7 @@ end
 -- You can either provide your own charset or the function will use
 -- a default one which is [A-Z].
 -- @param len Length of the string we want to generate.
--- @param charset Charset that will be used to generate the string.
+-- @param charset Charset that will be used to generate the string. String or table
 -- @return A random string of length <code>len</code> consisting of
 -- characters from <code>charset</code> if one was provided, otherwise
 -- <code>charset</code> defaults to [A-Z] letters.
@@ -261,8 +261,15 @@ function generate_random_string(len, charset)
   local ascii_A = 65
   local ascii_Z = 90
   if charset then
-    for i=1,len do
-      t[i]=charset[random(#charset)]
+    if type(charset) == "string" then
+      for i=1,len do
+        local r = random(#charset)
+        t[i] = sub(charset, r, r)
+      end
+    else
+      for i=1,len do
+        t[i]=charset[random(#charset)]
+      end
     end
   else
     for i=1,len do