fix leak in mock_server_t test utility

Author: ajdavis

tests/mock-server.c

@@ -264,6 +264,7 @@ mock_server_worker (void *data)
    mongoc_stream_close (stream);
    mongoc_stream_destroy (stream);
    bson_free(closure);
+   _mongoc_buffer_destroy (&buffer);
 
    RETURN (NULL);
 }