Fix logic for dropping old events in fed queue

Fixes infinite loops of

> logger.info("Invalid prev_events for %s", event_id)

Author: richvdh

synapse/storage/databases/main/event_federation.py

@@ -1432,7 +1432,10 @@ async def prune_staged_events_in_room(
 
             if room_version.event_format == EventFormatVersions.V1:
                 for prev_event_tuple in prev_events:
-                    if not isinstance(prev_event_tuple, list) or len(prev_events) != 2:
+                    if (
+                        not isinstance(prev_event_tuple, list)
+                        or len(prev_event_tuple) != 2
+                    ):
                         logger.info("Invalid prev_events for %s", event_id)
                         break