Don't create broken room when power_level_content_override.users does not contain creator_id.

changelog.d/5633.bugfix

@@ -0,0 +1 @@
+Don't create broken room when power_level_content_override.users does not contain creator_id.

synapse/handlers/room.py

@@ -752,6 +752,13 @@ def send(etype, content, **kwargs):
             if power_level_content_override:
                 power_level_content.update(power_level_content_override)
 
+            if creator_id not in power_level_content["users"]:
+                raise SynapseError(
+                    400,
+                    "Not a valid power_level_content_override, did not contain %s"
+                    % (creator_id,),
+                )
+
             yield send(etype=EventTypes.PowerLevels, content=power_level_content)
 
         if room_alias and (EventTypes.CanonicalAlias, "") not in initial_state: