typo

Author: jbergstroem

pymongo/_cbsonmodule.c

@@ -980,7 +980,7 @@ static int add_last_error(bson_buffer* buffer, int request_id, PyObject* args) {
 
  /* save space for length */
  document_start = buffer_save_bytes(buffer, 4);
- if (document_length == -1) {
+ if (document_start == -1) {
  return 0;
  }