core: fix crash in fork stress test

14:50:04 E mitogen: mitogen.fork.Stream('fork.7431') crashed
Traceback (most recent call last):
  File "/home/dmw/src/mitogen/mitogen/core.py", line 1287, in _call
    func(self)
  File "/home/dmw/src/mitogen/mitogen/core.py", line 758, in on_receive
    return self.on_disconnect(broker)
  File "/home/dmw/src/mitogen/mitogen/parent.py", line 370, in on_disconnect
    super(Stream, self).on_disconnect(broker)
  File "/home/dmw/src/mitogen/mitogen/core.py", line 721, in on_disconnect
    fire(self, 'disconnect')
  File "/home/dmw/src/mitogen/mitogen/core.py", line 162, in fire
    return [func(*args, **kwargs) for func in signals.get(name, ())]
  File "/home/dmw/src/mitogen/mitogen/core.py", line 1160, in <lambda>
    listen(stream, 'disconnect', lambda: self.on_stream_disconnect(stream))
  File "/home/dmw/src/mitogen/mitogen/core.py", line 1142, in on_stream_disconnect
    for context in self._context_by_id.itervalues():
RuntimeError: dictionary changed size during iteration

mitogen/core.py

@@ -1139,7 +1139,7 @@ class Router(object):
         return 'Router(%r)' % (self.broker,)
 
     def on_stream_disconnect(self, stream):
-        for context in self._context_by_id.itervalues():
+        for context in self._context_by_id.values():
             stream_ = self._stream_by_id.get(context.context_id)
             if stream_ is stream:
                 del self._stream_by_id[context.context_id]