Clear self.interface when switching

When switching and the interface isn't immediately available,
we should clear self.Interface as otherwise requests will
still be going to it.

lib/network.py

@@ -418,6 +418,7 @@ class Network(util.DaemonThread):
         if server already is our interface.'''
         self.default_server = server
         if server not in self.interfaces:
+            self.interface = None
             self.start_interface(server)
             return
         i = self.interfaces[server]