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

changelog.d/5633.bugfix

@@ -0,0 +1 @@
+Fix createRoom power_level_content_override behaviour to match the spec.

synapse/handlers/room.py

@@ -750,7 +750,14 @@ def send(etype, content, **kwargs):
                     power_level_content["users"][invitee] = 100
 
             if power_level_content_override:
-                power_level_content.update(power_level_content_override)
+                power_level_content = 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)