Avoid triggering FileMigrateError when possible

telethon/sessions/memory.py

@@ -42,6 +42,10 @@ class MemorySession(Session):
         self._server_address = server_address
         self._port = port
 
+    @property
+    def dc_id(self):
+        return self._dc_id
+
     @property
     def server_address(self):
         return self._server_address