Relax http.parse_form to allow forms without an action

Patch from nnposter: http://seclists.org/nmap-dev/2014/q3/384

scripts/http-csrf.nse

@@ -134,7 +134,7 @@ action = function(host, port)
         form = http.parse_form(form)
 
         local resistant = false
-        if form then
+        if form and form.action then
           for _, field in ipairs(form['fields']) do
 
             -- First we check the field's name.