Fix undefined `room_id` in `make_summary_text`

This would break notifications about un-named rooms when processing
notifications in a batch.

synapse/push/mailer.py

@@ -555,10 +555,12 @@ class Mailer(object):
             else:
                 # If the reason room doesn't have a name, say who the messages
                 # are from explicitly to avoid, "messages in the Bob room"
+                room_id = reason["room_id"]
+
                 sender_ids = list(
                     {
                         notif_events[n["event_id"]].sender
-                        for n in notifs_by_room[reason["room_id"]]
+                        for n in notifs_by_room[room_id]
                     }
                 )