only process admin commands if bot is in room

This isn't exploitable in any way, it just meant that the bot could
receive and run commands from users who were already in the admin room
despite the bot not being in the room, which also means the bot would be
unable to send response messages.

Now, the bot will simply ignore admin room messages if the bot isn't in
the admin room.

src/service/rooms/timeline.rs

@@ -536,6 +536,11 @@ impl Service {
                         if to_admin_bot
                             && !from_admin_bot
                             && admin_room == pdu.room_id
+                            && services()
+                                .rooms
+                                .state_cache
+                                .is_joined(admin_bot, &admin_room)
+                                .unwrap_or(false)
                         {
                             services().admin.process_message(body);
                         }