Fix timeout function

Turns out deferred.cancel sometimes throws, so we do that last to ensure
that we always do resolve the new deferred.

synapse/util/async_helpers.py

@@ -456,11 +456,12 @@ def timeout_no_seriously(deferred, timeout, reactor):
 
     def time_it_out():
         timed_out[0] = True
-        deferred.cancel()
 
         if not new_d.called:
             new_d.errback(DeferredTimeoutError(timeout, "Deferred"))
 
+        deferred.cancel()
+
     delayed_call = reactor.callLater(timeout, time_it_out)
 
     def convert_cancelled(value):