asyncore: introduce a new 'closed' attribute to make sure that dispatcher gets closed only once.

In different occasions close() might be called more than once, causing problems with already disconnected sockets/dispatchers.

Lib/asyncore.py

@@ -220,7 +220,7 @@ class dispatcher:
 
     connected = False
     accepting = False
-    closing = False
+    closed = False
     addr = None
     ignore_log_types = frozenset(['warning'])
 
@@ -393,6 +393,8 @@ class dispatcher:
                 raise
 
     def close(self):
+        if not self.closed:
+            self.closed = True
             self.connected = False
             self.accepting = False
             self.del_channel()