Don't create broken room when power_level_content_override.users does…

… not contain creator_id. (#5633)

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

@@ -579,6 +579,18 @@ def create_room(self, requester, config, ratelimit=True, creator_join_profile=No
 
         yield self.event_creation_handler.assert_accepted_privacy_policy(requester)
 
+        power_level_content_override = config.get("power_level_content_override")
+        if (
+            power_level_content_override
+            and "users" in power_level_content_override
+            and user_id not in power_level_content_override["users"]
+        ):
+            raise SynapseError(
+                400,
+                "Not a valid power_level_content_override: 'users' did not contain %s"
+                % (user_id,),
+            )
+
         visibility = config.get("visibility", None)
         is_public = visibility == "public"
 
@@ -621,7 +633,7 @@ def create_room(self, requester, config, ratelimit=True, creator_join_profile=No
             initial_state=initial_state,
             creation_content=creation_content,
             room_alias=room_alias,
-            power_level_content_override=config.get("power_level_content_override"),
+            power_level_content_override=power_level_content_override,
             creator_join_profile=creator_join_profile,
         )