Avoid raising errors due to malformed IDs in get_current_hosts_in_room

Handle malformed user IDs with no colons in `get_current_hosts_in_room`.
It's not currently possible for a malformed user ID to join a room, so
this error would never be hit.

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/13748.misc

@@ -0,0 +1 @@
+Avoid raising an error due to malformed user IDs in `get_current_hosts_in_room`. Malformed user IDs cannot currently join a room, so this error would not be hit.

synapse/storage/databases/main/roommember.py

@@ -1044,6 +1044,8 @@ async def get_current_hosts_in_room(self, room_id: str) -> List[str]:
             # We use a `Set` just for fast lookups
             domain_set: Set[str] = set()
             for u in users:
+                if ":" not in u:
+                    continue
                 domain = get_domain_from_id(u)
                 if domain not in domain_set:
                     domain_set.add(domain)
@@ -1071,6 +1073,7 @@ def get_current_hosts_in_room_txn(txn: LoggingTransaction) -> List[str]:
                     c.type = 'm.room.member'
                     AND c.membership = 'join'
                     AND c.room_id = ?
+                    AND server_domain IS NOT NULL
                 /* Group all state events from the same domain into their own buckets (groups) */
                 GROUP BY server_domain
                 /* Sorted by lowest depth first */