Fix error looking at empty port in sp plugin.

Author: Andreas Richter

src/s2repoze/plugins/sp.py

@@ -508,6 +508,8 @@ def identify(self, environ):
             binding = BINDING_HTTP_REDIRECT
         else:
             post = self._get_post(environ)
+            if post.list is None:
+                post.list = []
             binding = BINDING_HTTP_POST
 
         try: