sudo_module_register_loghandler: clear sudo_type_LogHandler on error

Also add comments about PyModule_AddObject stealing a ref on success.

plugins/python/python_convmessage.c

@@ -91,6 +91,7 @@ sudo_module_register_conv_message(PyObject *py_module)
         goto cleanup;
     }
 
+    // PyModule_AddObject steals the reference to py_class on success
     Py_INCREF(py_class);
     rc = SUDO_RC_OK;