issue #498: prevent crash on double 'disconnect' signal.

Fixes:

ERROR! [pid 1096] 23:31:48.363215 E mitogen: _broker_main() crashed
Traceback (most recent call last):
  File "/home/dmw/src/mitogen/mitogen/core.py", line 2917, in _broker_main
    self._loop_once()
  File "/home/dmw/src/mitogen/mitogen/core.py", line 2875, in _loop_once
    self._call(side.stream, func)
  File "/home/dmw/src/mitogen/mitogen/core.py", line 2860, in _call
    stream.on_disconnect(self)
  File "/home/dmw/src/mitogen/mitogen/parent.py", line 1161, in on_disconnect
    super(Stream, self).on_disconnect(broker)
  File "/home/dmw/src/mitogen/mitogen/core.py", line 1534, in on_disconnect
    fire(self, 'disconnect')
  File "/home/dmw/src/mitogen/mitogen/core.py", line 390, in fire
    func(*args, **kwargs)
  File "/home/dmw/src/mitogen/mitogen/parent.py", line 1794, in <lambda>
    func=lambda: self._on_stream_disconnect(stream),
  File "/home/dmw/src/mitogen/mitogen/parent.py", line 1810, in _on_stream_disconnect
    routes = self._routes_by_stream.pop(stream)
KeyError: mitogen.ssh.Stream('ssh.localhost:2236')

mitogen/parent.py

@@ -1812,9 +1812,15 @@ class RouteMonitor(object):
 
     def _on_stream_disconnect(self, stream):
         """
-        Respond to disconnection of a local stream by
+        Respond to disconnection of a local stream by propagating DEL_ROUTE for
+        any contexts we know were attached to it.
         """
-        routes = self._routes_by_stream.pop(stream)
+        # During a stream crash it is possible for disconnect signal to fire
+        # twice, in which case ignore the second instance.
+        routes = self._routes_by_stream.pop(stream, None)
+        if routes is None:
+            return
+
         LOG.debug('%r: %r is gone; propagating DEL_ROUTE for %r',
                   self, stream, routes)
         for target_id in routes: