PYTHON-1599 Fix unchecked return value in cmessagemodule

Author: ShaneHarvey

pymongo/_cmessagemodule.c

@@ -1199,6 +1199,10 @@ _batched_op_msg(
  }
  /* Save space for size */
  size_location = buffer_save_space(buffer, 4);
+ if (size_location == -1) {
+ PyErr_NoMemory();
+ return 0;
+ }
 
  switch (op) {
  case _INSERT: