PYTHON-685 - Fix rare resource leak in _cmessage

Author: behackett

pymongo/_cmessagemodule.c

@@ -1112,8 +1112,10 @@ _cbson_do_batched_write_command(PyObject* self, PyObject* args) {
  */
  buffer_update_position(buffer, sub_doc_begin);
 
- if (!buffer_write_bytes(buffer, "\x00\x00", 2))
+ if (!buffer_write_bytes(buffer, "\x00\x00", 2)) {
+ buffer_free(new_buffer);
  goto cmditerfail;
+ }
 
  result = _send_write_command(client, buffer,
  lst_len_loc, cmd_len_loc, &errors);