Prevent "producer not unregistered" message (#5009)

changelog.d/5009.bugfix

@@ -0,0 +1 @@
+Clients timing out/disappearing while downloading from the media repository will now no longer log a spurious "Producer was not unregistered" message.

synapse/rest/media/v1/_base.py

@@ -191,6 +191,10 @@ def respond_with_responder(request, responder, media_type, file_size, upload_nam
         # in that case.
         logger.warning("Failed to write to consumer: %s %s", type(e), e)
 
+        # Unregister the producer, if it has one, so Twisted doesn't complain
+        if request.producer:
+            request.unregisterProducer()
+
     finish_request(request)