PYTHON-1647 Fix memory leak in OP_MSG C extensions

(cherry picked from commit d571ac0)

pymongo/_cmessagemodule.c

@@ -762,6 +762,7 @@ static PyObject* _cbson_op_msg(PyObject* self, PyObject* args) {
     Py_XDECREF(iterator);
     buffer_free(buffer);
 bufferfail:
+    PyMem_Free(identifier);
     destroy_codec_options(&options);
     return result;
 }