fix segfault when clearing an unset oneof in Python
https://github.com/protocolbuffers/protobuf/issues/10461

PiperOrigin-RevId: 472891559
diff --git a/python/message.c b/python/message.c
index 47b00e8..e67bf23 100644
--- a/python/message.c
+++ b/python/message.c
@@ -1361,7 +1361,7 @@
   }
 
   if (o) f = upb_Message_WhichOneof(self->ptr.msg, o);
-  PyUpb_Message_DoClearField(_self, f);
+  if (f) PyUpb_Message_DoClearField(_self, f);
   Py_RETURN_NONE;
 }